Wont let me change profile picture

As stated, if you have changed you date of birth to show you are over 18 (20 in South Korea and Japan) then it will not work. Xbox accounts only recognise the date of birth entered on creation and you will have to wait for your account to naturally propogate
to adult.

Hi there, sorry you are having issues. Did you happen to change the birth date on the account recently to make it an adult account? If so this method doesn't work and will still recognise the account as a child account.

Long into your account at account.microsoft.com and go to the family tab.

This is will determine if your account is under someone account acting as the parent.

If you are you need to have the parent log into the account and remove from the family or update your settings

You can also update your age of your account by contacting Xbox Support and providing the information they request to prove you are over the age of 18 (20 in Korea) with a debit/credit card.

how do i change my birth date

You have to be 18 or over. Changing your age after the fact won't work. This is to prevent kids from just changing their age to bypass things. Whatever age you used when you set up the account is the one that's permanently on file. You can change you age
anytime after the fact but the system won't recognize it.