I've looked on the internet and it says you can change the age of your Google account if you have a credit card. I entered my DOB incorrectly, it says I'm a year younger than I am. However I can' find any page to enter any card or verification details?
This is an answer which does not provide a solution but declares that there is lack of a solution at present time.
After checking with Google Product forums, Gmail support, youtube support and Google + support there is currently no way to change your date of birth on a Google Account.
The lame way to change it is to delete and create a new Google Account.