Any player can get this injury, but some are more prone. There are some MLB players who have problems with the hip flexor (Chipper Jones) so it doesn't always go away with age. The thing that does go away is the higher risk of pulling away from the soft bone at the growth plate. That hardens as boys hormones kick in (beard growth is the first indication that the growth plate is closing.) I suspect my son will always need to be very careful and do the stretching and strengthening exercises prescribe by the physical therapist.
The kids I know who have received this injury got it from a jarring collision: foot hit a base or player on slide and jacked the hip suddenly, or catcher collision at the plate. Any player who slides could get a hip flexor injury. If they have pain on rotation, they should certainly shut down until they've healed the area, visit a PT, and do the exercises given to increase flexibility.