It doesn't take Merlin long to figure out that he doesn't age the same as everyone else.
He can, if he wants—and he does, living entire lifetimes before cycling back and moving on, stretching it out just to see how far it goes. He gets all the way to 247, just once, stubbornly testing his limits until his withering body becomes so frustrating that he abruptly ages himself down to ten and spends a few years in an orphanage, spry and carefree and inciting the other kids to raise hell.
Adapting to the future is easy. He has nothing but time, and Merlin has always been a quick study, anyway. He skips huge swaths of history, once he learns how; the first time he tries, he ends up jumping straight from 1430 to 1876, and he's horribly disappointed once he finds out he missed the Renaissance.
He knows, somehow, that the world he's seen so far isn't the same world that awaits Arthur's return. That's why he doesn't feel very bad about missing it. Fine, yes, it would have been nice to meet Shakespeare, and perhaps he should have done more to avert those ever-escalating wars in the 20th century, and perhaps he should have tried harder to make deeper connections and keep true friends—but it's so difficult to focus his energies productively, when the biggest part of him is still so caught up in waiting.
And then, something like 1500 years after Merlin put Arthur in that lake, when the human race has grown by the billions, and the forests have shrunk and the seas have risen, and one of Albion's infant colonies has taken over as the primary global superpower (and really, how embarrassing), Merlin is just taking an evening to himself, watching football in his current favorite pub, when he turns around and spills half his pint down the front of an expensive-looking shirt.
"Bloody hell, sorry!" Merlin says. (He loves modern cursing. He especially loves how fuck has evolved into so many parts of speech; he remembers back when it was mainly used in its literal sense—and that was very practical, to be sure, but a lot less fun.)
"Oi, watch it!" snaps the man inside the shirt, and a jolt of recognition goes through Merlin's whole body. He drops the pint glass.
"You're back," Merlin says dully, staring into bright-blue, furious eyes. "How did I miss it? Oh, erm, sorry about the shirt, Arthur. And your shoes."
Arthur gives him an imperious raise of his eyebrow—achingly familiar—and looks down at the shattered glass and spilled cider ruining his shiny shoes. "Right then," he says, crossing his arms. "I'm not generally known for filing lawsuits against clumsy oafs who ruin my wardrobe, but I'm willing to make an exception if you keep at it, so—do I know you?"
Merlin beams, feeling everything in him burn with the warm, gold glow of rightness and destiny and home. "Merlin," he says, holding out his hand. Which Arthur ignores.
"So I don't know you," he continues. "Yet you seem to know me. Or my name, at least. Did you follow me here? Did my father send you?"
"Oh, bollocks, Uther's back?" Merlin rolls his eyes. "Figures."
"Well, no, he's probably still in Japan," Arthur says, narrowing his eyes and tilting his head, like he can figure out what's going on if he can just bring Merlin's face into focus. Merlin can't stop smiling, even though he can tell he looks absolutely mad. "Your eyes are... have I seen you? When..."
"A long time ago," Merlin says, stepping forward and slipping his palm boldly behind Arthur's neck. Arthur lets him, steps in to match him, closes his eyes.
"Everything you've done. I know now," Arthur murmurs, eyes still closed. "I know, how do I... Merlin—"
"I'm here," Merlin says, gently, and for all that he's imagined this moment—whirlwinds, trumpets sounding, fanfare and fireworks—it's almost anticlimactic, like waking up from a dream and feeling reality slip quietly back into place.
"By the gods, I..." Arthur takes a shuddering breath, and Merlin watches his red lips part for it, watches the rise and fall of his chest and tracks the color in his cheeks. Arthur is so extremely alive. "I can't believe..."
"Yes," Merlin agrees, bringing his other hand up to slide into Arthur's hair. He's always wanted to do that—just touch for no reason—and now's as good a time as any. 1500 years was long enough to wait.
"I can't believe... that all this time I've needed you and you've been in the tavern."
"I haven't—hey!" Merlin draws back, indignant. "I haven't been here your whole life, you prat! I just come here on weekends!"
"Nothing changes," Arthur says, throwing his hands up dramatically. "It would be too much to hope for you to have learned to be actually useful, after a full millennium of free time."
"Well, you asked me not to change," Merlin says, and he feels the prickle of tears at that, because it's not like that memory has ever faded. It still feels to him like he lost Arthur yesterday, no matter how many lifetimes he spends without him. "How would you even recognize me, if I suddenly became punctual and accommodating?"
"I'd recognize you no matter what, you idiot," Arthur scoffs, and Merlin is opening his mouth to point out that actually, Arthur hadn't recognized him, at first, when Arthur suddenly reels him in by his scarf and kisses him so hard that Merlin is knocked back against the bar.
It's surprising, but it's not; they've always fit together, they exist to fit together, and this is just another facet of that, after all.
(Plus, it's not like Merlin hasn't thought about it, during some of his more optimistic lifetimes—a cherished fantasy to ease the ache of his centuries of isolation. He's had a lot of time to imagine this. He's got plans.)
The kissing, though, that's good enough for now—more than good enough, with Arthur's breath in his mouth and Arthur's arms tight around his waist. Merlin rests his fingers reverently on his jaw, guiding and stroking, and ignores the pain from where Arthur's body is pressing his back into the sharp edge of the countertop.
"Take me home," Merlin gasps when they break apart.
"Fast," Arthur says breathily, pressing their foreheads together and grinning.
"1500 years," Merlin says. "Not fast enough. Take me home."
"Yes, sire," Arthur says, and kisses him again.