Susan Pevensie stops aging on her twenty-eighth birthday.
She's not sure why, exactly. But that's the oldest she ever got to be in Narnia, and now Narnia is gone. She wonders, perhaps, if something broke somewhere when Narnia ended, like some eternal clock popped a spring, and the result was that she is stuck at age 28.
She knows day she turns 28 that she's not ever turning any older. She doesn't know how she knows, but she does. She supposes it's not so bad, really. It's a good age — too old to be really young, and too young to really be old. She hasn't thought of herself as vain, not in a long while, but when she looks in the mirror, she's a tiny bit pleased that there's not a wrinkle on her face, and her hair has no grey, and that's the way it's always going to stay.
Of course, there's the whole immortality thing to adjust to, but the young man she meets in the nightclub on her birthday turns out to be very helpful on that subject.
"It can get very boring, it's true," Maglor says, as he leans in close to light her cigarette for her. "But you can always go insane for a century or two. It helps pass the time, I've found."
She doesn't go insane (not right away, in any case). It turns out that a chance meeting with this man — elf, actually — give her a companion to while away eternity with. Maglor teaches her how to survive as an immortal in a world filled with humans who come and go in the blink of an eye, how to fit in and not stand out, how to not be noticed as eternally young, how to care for and love children who'll leave you behind, and how to let go.
They spend the first several decades traveling the world, observing and staying detached, hiding in plain sight among the crowds of mortals — sometimes working in the slums with the poor, sometimes with the rich and famous, glittering in the crowds of beautiful people, sometimes in quiet, middle class neighborhoods, working as shopkeepers or teachers, making friends who they'll eventually leave behind.
One night Susan wakes to find Maglor staring at her, wide awake in the bed beside her.
"What is it?"
He just shakes his head. "I still don't understand. You're a human. You're not supposed to live forever. And I'm supposed to be alone."
She leans in close, her forehead touching his, and cards her fingers through the tangles of his dark hair. "Maybe neither of us were meant to be alone. We were just waiting to find each other."
He sighs, breath ghosting over her skin as he presses his face in her neck. "Maybe. Maybe."
Susan doesn't think he's convinced, and really, she doesn't blame him. It doesn't feel like much of a life they're living, even though they're together. They're observers to a world that keeps turning, keeps moving, and it seems like life is passing them by.
She climbs out of bed, leaving him to sleep, and sits down at her desk, to make a list.
When he wakes the next morning and comes to find her, she greets him with a tired smile. "Pack your things. We're going to find a life."