Account age restrictions

Once an account is created the age can not be changed. You will have to wait until the account age reaches the appropriate age for restricted games.

Are there any age restrictions on your account?

So you can sign into your account at account.microsoft.com/family and it will allow you to set restrictions on what he can purchase as far as age restrictions and if your password would be needed to allow him to purchase things above what is restricted age
wise. if you have your credit/debit card on his account you can remove it and from the family website it will allow you to add money to his account to make purchases with and within the age restriction this way you can limit what types of games he is allowed
to purchase to age appropriate/content restricted games.

EA has age restrictions in place. If the age of the account is set wrong and it's under 13, then it won't allow you to play. There's no way to change the age on an account.

I've tried everything

I assume you are trying to change your age in order to get around parental restrictions?

If so, even if you could change the age on your account, it won't accomplish getting around parental restrictions.

Your Xbox account will always remember the age originally entered in at account creation so even if you change your age from that if a child to an adult for instance, you will still have parental restrictions.