Google Plus and age restrictions

I need some Google help. My 12 year old son has had a Google account for a couple of years. His cousin sent him a link to sign up for Google Plus. He followed the on screen instructions which included entering his date of birth (something that I didn't have to do 2 years ago when I set up the account for him - or if I did there wasn't an age restriction at the time). He can now no longer login to Gmail and gets the following message.

The Google Account for xxx@gmail.com has been disabled.You will not be able to sign in to this account or use it to access any Google products or services.You do not meet the age requirements for a Google Account. This account will be deleted in 27 days unless the birthday you entered was incorrect and you submit proof that you are 13 years old or older. Learn more.If you are 13 years old or older, click here to start the account unlock process.

Any ideas? Or have the Ts&Cs changed since G+ kicked off and now you have to be 13 to even have Gmail?