Changing age for a child account

Support is correct. It's not possible to change the age of an account once it has been created and change an adult account to a child account. In order to be able to use restrictions for him, you would have to create a brand new, child account for him and
your account would be the parent account. Nothing can be transferred between accounts so he would have to start over.

Are you sure you can't change the age of an account? I just went into my account on live.com, and went under personal info. I changed my age to the age of my child. When I hit "save", it prompted me that I first had to revoke consent for my child accounts.
I didn't continue, but it appears that if I do revoke consent, then I will have successfully changed my age. Then I'd have my account with a child's age, and I could create a new account and make my old account a child of the new account...

Changing the age under your profile data cannot change your xbox account from child to adult. If you have registered an account under 18+ of age in xbox live service that is a child account

Changing your birthdate under your MS profile can't change your account from child to adult. If you have registered an xbox account under 18+ of age it will remain a child account until the proper age is reached

There is not a way to change your child's account from child to adult until the age of the account is 18. What specific settings are you trying to change?

If you're on a child account, you will have to make the purchase on a 360 console. Changing the age of your Microsoft account does not change the age of your Xbox account. That cannot be changed.