I think age is important to some people. Where I come from, a lot of kids don't get to be kids very long, so age doesn't matter as much. But 18 was the age when you couldn't get Reaped anymore, so I guess it might have been a safer bet to date someone older than that, so you knew they weren't going to die.
Except that's not how things ended up working out, anyway. I don't know what the rules about age are in other places.