I need some Help!

I'm Outlining a story, and I have been thinking,

My story is about three people (one girl and two boys) Trying to stop a Demon war that has broken out on earth, while the girl deals with finding out she is half Healer.
(They are like Engel but a bit less powerful and they look after earth.)

My problem is I can't decide to make them Teenagers (17 - 19) oder young adults (22-24) I know its not a big age different but it makes a differences in the story.

So I was wondering what Du guys thought, I'm always looking for other opinions. If Du could help me I would appropriate it.

Thank Du :)