I was thinking similar. They grew up normally, although their parents thought it was strange they never got sick (I imagine their regeneration protects them from any illness, since that makes sense), and when they finished elementary school (or middle school if you want. We don't have those here, so I just say elementary), ready to enter high school, that's when they get a letter from the school.

Or it could be when they're entering middle school. I don't know the ages of middle school though. I don't think they should be too young...