We had the same situation. My 11 year old daughter has had a Gmail account for many years and started using a couple years ago. Everything was fine until she tried to accept a Google+ invite and was locked out. Unfortunately this was more than just losing a few silly emails as this is her primary account for lots of school activities.

My wife found the credit card verification link and got the account reinstated. I wish Google+ would make it more clear or just disallow underaged accounts and not invalidate the whole Google account.