When they had been—had looked—the same age, Merlin had been just Merlin. Just his manservant; just a man with elbows that pointed awkwardly and clumsy feet and ears too large for his head, who smiled a regular sort of smile when he was pleased. Certainly not the kind of smile to make something in Arthur's chest stutter.
Arthur remembers being young. Youth was: fluidity of muscle and joint, easy long nights, sharp eyes. Though Arthur keeps himself as fit as possible (at least an hour every day in training) he cannot fight the pull of years, and now—he is not young.
He is not old either. His hair still grows thick golden-gray, his reach is not compromised, his muscles swell when he clenches. Long nights have gotten harder, but not impossible. Arthur is nowhere near old enough to worry yet.
But he is old enough to know better and, he would have thought, old enough not to fumble into something that moves inside him with slow inexorability and darting fever. He has known love, has been familiar with it and all its trials from beginning to end; he is too old for it.
Merlin is immortal. They discovered this in a series of fits and starts.
1) Merlin, too addled with exhaustion to think straight, throws himself in front of a dagger for Arthur. When it pierces his back, the force of the blow takes him into Arthur's arms. Arthur clutches him, sinking to the ground with his weight, feeling sticky hot blood spread under his hands. He cannot speak, he cannot think, except that Merlin's grip on his shirt is weakening, he is coughing, he is saying something and he is limp, he is a useless dead thing in Arthur's embrace, he is a corpse.
Until, four hours later, he opens his eyes.
2) Uther tries to burn him; he burns.
3) Uther tries to cut off his head; it rolls on the ground.
It would have taken them much longer to notice had Merlin not left. The stability of his age would not have been so shocking if they had been taking it for granted, maybe laughing like oh, Merlin, he never seems to grow up. But he does leave.
4) Merlin leaves and years pass, and Arthur and Gwen and Gaius and everyone all show the signs of it. Arthur grooms a beard; Gwen develops small, delicate lines under her eyes that crinkle when she laughs; Gaius grows smaller. Everyone ages.
So when Merlin comes back, looking the same as the day he first called Arthur a prat (except for the strangely-crafted clothing and the crackling power under his fingernails), they are all struck by the strangeness—that a man nearing thirty should appear not a day older than nineteen—the unnaturalness.
It is one thing to be invincible; It is quite another be eternally young.
Merlin looks exactly the same. Except that he doesn't. Awkward elbows have become hands, long and clever, flowing into lean white arms. Clumsy feet have become fine-boned with ankles that are perfect arches. The ears—the ears are the same, only now Arthur wants to breathe dark words into them and feel Merlin shiver against his body.
If only he had wanted Merlin when he was young, when they were both—it might have been possible.
Arthur is forty-four; his wife has come and gone away. He is a King who has forged a nation from the squabbling leftovers of Roman expansion, who answers to no one, who commands the greatest sorcerer to ever live (and live and live and live), and yet he is being brought down low by lust like any common beast.
He is ashamed when he looks at Merlin
...snoring, Merlin tripping, Merlin bowing, Merlin stirring potions, Merlin swimming in the lake, Merlin stroking the kittens, Merlin pulling a storm from the sky to drive the Saxon ships away, Merlin lolling tipsily in Arthur's favorite chair, Merlin eating grapes, Merlin shivering in the cold early spring, Merlin...
and feels it burn in him, ashamed as he fucks his hand in the early morning, the mid-afternoon, the late, late night, ashamed at the shallowness of his own desire and ashamed of the filthy things he dreams up.
He is not ashamed when Merlin smiles at him, and he feels, finally, like the center of a world of hope: then he is just tired.
Merlin comes to him one evening while he's sitting, lost in thought, in front of his hearth.
"Arthur," he says, sounding impatient, leaning against Arthur's doorjamb like he might be posing deliberately. "Are you done yet?"
Arthur raises an eyebrow, echoes, "Done?"
Merlin gestures widely. "You know. With this thing and the sulking and the noble repression and all that rot. Can you please be done?"
"I'm sure I don't know what you mean," says Arthur. Sadly, he means it. "Speak sense, Merlin."
The look Merlin gives him is deeply unfair, since he's the one blathering cryptically.
"Oh, fine," Merlin huffs after a moment of intense judgmental staring. "I have to do everything around here."
He comes fully inside the room and closes the door, latches it, then crosses to Arthur and straddles his lap like a slattern, thighs wide over Arthur's hips, knees tucked between the seat and the armrests. Arthur jerks back in shock, then up into the warmth of Merlin's body, then back again in horror at himself, mind looping stuck between what the fuckand yes yes yes and no no no.
"Stop it," Merlin says sternly, and kisses him.
His mouth is lush and hot. Arthur must black out for a moment, because the next thing he knows, he's got both hands in Merlin's hair, tight, and is licking Merlin's teeth. He wants—he wants to pull away, to ask for an explanation—is Merlin enchanted, oh hell—but he can't, he just can't. All he can do is hold Merlin close and kiss and kiss and kiss him.
The strength with which he's gripping Merlin must be painful, it makes Arthur's muscles shake, but Merlin just sucks his tongue and grunts, grinding against Arthur, body taut and burning like living lightning that strikes right to Arthur's core. Arousal races through him, fills his cock with blood so fast he gets dizzy, but it feels so good, so good to push it up into the pressure of Merlin's weight. He hasn't felt anywhere near this way in a long, long time.
"Fuck," Merlin says, all muffled because he's trying to break from the kiss but Arthur keeps chasing his lips down. "Fuck, let's. Bed."
"Merlin," Arthur says, grasping at his last shred of self-control, "I don't—"
"I do," Merlin interrupts. He grins as he slides out of Arthur's lap, grabbing Arthur's vest and dragging him out of the chair—too eager, Arthur's knees protest. They stumble together to Arthur's bed, where Merlin shoves him down again and climbs back on top. He braces his hands on Arthur's chest and just—shoves his cock against Arthur's, wanton rutting, head hanging down and mouth open. Their clothes are still on, but Arthur thinks he could come from this like an unblooded youth.
Arthur rubs his hands up Merlin's thighs, curving them over the thin width of his hips, bony, palms Merlin's arse to increase the force between them. Merlin grunts again and scrambles at Arthur's tunic, bunching it up under Arthur's armpits so he can scratch blunt fingernails through the thick hair on Arthur's chest. Arthur wants to pull it back down, to cover the softening of his middle, his age belied.
He doesn't; Merlin's eyes are on Arthur's face, dark in ways Arthur's never seen, even in his fantasies.
Helpless, Arthur curls upwards for another kiss. Merlin denies him, instead yanking Arthur's shirts over his head the second he can then fumbling his own off, all the while his legs working to keep them rocking hard together. Merlin's torso is narrow and pale, smattered with dark, wiry hair around his nipples and down his abdomen. He has no scars.
Arthur snatches at Merlin's neck and drags him down. They kiss again, finally, sloppy and open-mouthed, Arthur tonguing Merlin's bottom lip, biting down on it just to feel it give softly between his teeth. Saliva dampens his beard. Merlin's skin is soft and firm, overwhelming.
Arthur rolls them over, presses Merlin into his bedding with his whole body, and Merlin gives so easily for him. Flexible: his mouth and his legs and his arms falling open for Arthur to crowd as close as possible. He wants to burrow inside and live forever with Merlin in his perfect skin.
Merlin wraps his long legs around Arthur's hips and arches up, says, "Trousers."
"You're the sorcerer," Arthur mutters into Merlin's throat.
"Ha!" says Merlin. "Lazy."
Arthur means to respond in kind, but Merlin disappears their trousers and—
Their cocks catch, slide together, sticky with pre-come. Merlin gives a wet gasp against Arthur's cheek and drags his hands through the sweat on Arthur's back as Arthur shudders.
"Merlin, Merlin," he groans.
"Yeah, now, just, uh," Merlin groans back, pushing Arthur over, away, so he can flip onto all fours. He looks at Arthur from the corner of his eyes and stretches, the sinful, nubile length of him—slim and unbelievable in Arthur's bed. His grin ruins the effect a moment later, but it nevertheless sends jolts through Arthur when he says, playful, "Fuck me?"
Arthur's fingers look coarse and old slicking Merlin up. Merlin whines, shoving onto them impatiently. Arthur nips at Merlin's shoulder blades and rubs his beard along the length of Merlin's spine. He watches the pink clench of Merlin's hole as he scissors two fingers inside, mesmerized. Oil drips over his palm and wrist as he trickles it between the spread cheeks of Merlin's arse, making it good and wet so he can twist a third finger into the tight space.
Merlin drops to his elbows at the sensation, trembling around breathless words.
Arthur doesn't ask. He nudges the purple head of his cock against Merlin's arse, rubs it through the remnant oil between his arse cheeks, then shoves, slowly, inside.
Merlin, limber as gypsy dancer, goes readily when Arthur pushes his knees further apart, rucking up the heavy velvet blanket. He's got Merlin spread low, head hanging between his shoulders so his forehead rests on the bed; Merlin, all stupidly new-forever-beautiful, writhing back onto Arthur's cock making filthy sounds like fuuuuuck yeah yea-ah in in in hard oh.
The snug fit of his cock on Merlin's body is a kind of pleasure Arthur has never known. He trembles on the end of each thrust, his wiry dark pubic hair scratching at Merlin's arse. He wants to pace himself but he just fucks in hard and fast, one calloused, age-spotted hand on Merlin's left thigh, the other on his lower back. Arthur hauls him backward as he pushes forward; their skin slaps together.
Until it's over, suddenly, orgasm rushing out of his bones and Arthur is twitching as he spills. Merlin twists one hand down to his cock and works himself off as Arthur slumps to the side, a few short pulls and he goes rigid and comes all over Arthur's duvet. Arthur watches: the way his eyelids flutter, his red lips, his bony knuckles as semen drips over them, his eyes. Merlin is watching him back.
Arthur knows how he must look. Out of breath, red in the face, sweaty and worn. Old.
Merlin smiles. "I have loved and will love you for longer than you can imagine."
"Oh," says Arthur, shorter of breath than moments before.
He is too old for this.
Still. When Merlin smiles...
When Merlin smiles, as fresh-faced as the day they met, that same mocking gleam in his eyes, Arthur feels young again. Not a day over nineteen.