They'd be old enough to have children of their own. Technically, Luke and Leia are old enough to be parents to babies, but Leia isn't looking at that yet (and spoke a lot about adoption) and Luke...

Kind of knows he'll never have his own. There's thousands of orphans and at least some of them will be Force sensitive and need someone to guide them. He wants to find them and give them what his father never got.

"I've been told I'm pretty hard to pin down," he murmurs softly.