I ran into an issue where a game decided I was only 18, and then wanted to verify that I did put in the right brithdate. I basically found a method to change my birth date for my iTunes account, which should prove helpful for some.
2) Select Manage your account, which prompts you to log in.
3) Once signed in, select "Password and Security" option on the left.
4) Then use the drop downs to change your brith day, month, and year if needed and hit save.
Voila! Hopefully someone finds this useful.