"finished developing" is a stupid red herring. Our bodies and brains never stop changing. There is literally no point of "finished developing"
Our brains are largest around age 13. We get adult brains along with our adult bodies during puberty. This is a well-known scientific fact.
If anyone is "insulated from the real world" that was a choice society made. Was it the intentional infantilization of young adults? Keeping them locked out of adult society means they stay mentally children. We created this problem. We can remove it.