Current age restriction to join Google Plus is 18. You must be above 18 to join Google plus. But Google has decided to decrease this age restriction limit to 13.
According to Google Plus team, Google Plus will be open for all from 31/July/2011 and from same date age restriction will also decrease to 13.
This is going to be more fun with Google Plus. Facebook already have age restriction of 13.
This feature is not available of your account.
You must be over a certain age to use this feature. Learn more.