All Gmail accounts must be owned by a real person (even an account for a business) and since Google does not support all the paperwork involved for COPPA compliance (http://www.coppa.org/) that person must be 13 years old or older (the limit is higher in some countries). Google must comply with those rules and while it is not a new rule recently the enforcement has increased. But because e-mail addresses do not have to correspond to real names, Google can not guarantee a match between a person verifying account ownership and the account itself. Google must simply document that the stated owner of the account is 13 or older. For age-related locks, that verification is sufficient to unlock the account.