The two questions you need to answer in order for them to stay

Whenever someone, old or young, wants to join a group they will first determine the answer to two important questions. 

Is this group safe?
Can I make connections?

If a youth group, church or social club nails these two questions then they'll have a far greater chance of joining and yay sticking around.

This is why I stress connecting conversations with my leaders and, if pushed, would rather a group of girls over a group of all blokes.