Age settings on account

It doesn't matter if you change the age of an account after

The qualifications is set by the original age of the account which is set in stone

Some settings for Microsoft accounts are set by the age on the account, to comply with laws for different regions, and that age can vary by region. So based on the age set for the account, that will set certain permissions by default to certain settings,
and will need an Adult aged account assigned to your Microsoft Account family to make those changes legally.

So you can check your
Microsoft Account and make sure that your age is set properly, if you're of an adult age, and

possibly have to go through the Age Verification process.

Or you could alternatively, you could have your
parent's Microsoft Account claim you as a child in the Microsoft Account family, and then make those changes for your account from the Privacy and Online Safety Settings.

i Am extremely frustrated with this, i bought my brother Battlefield 3 and you apparently need to be 18 or over to play online. i basically just wasted about 50 dollars on a game that he cant use...

There is no way to change the age of an account once it's created. If the account is 18 or over you can promote the account to an adult account -
support.microsoft.com/.../945418

Are you part of a Family Gold Plan? If yes and you are not the primary account holder then your account is automatically assigned as a child account. It is a known issue with the FGP and the only workaround at this time is to
remove yourself from it, but that would also mean if you want to access XBL, for online matchmaking, you would need to purchase a seperate subscription for the account. If no then it is possible you entered the wrong information in your birthdate section when
you created your account. If that is the case you only have two options; a) wait until the account reaches 18 years of age or b) create a new account with the correct information entered.

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Just an FYI nothing is transferable from one account to another.