There are combination of things you need to do.
You need to first change the age on your profile.
https://account.microsoft.com/profile
Sign in to Your info on the Microsoft account website, choose Edit your personal info, and select the right birthdate. When you’re done, select Save.
Next, switch to a local account, then sign in again with your Microsoft Account:
Open Start > Settings > Accounts > Your info
Click sign in with a local account instead
Enter your current password
Click Next
Enter a name, password, confirm it, enter a hint
Click Next
Click Sign out and Finish
Sign into the account
Open Start > Settings > Accounts > Your info
Click sign in with a Microsoft account instead
Enter the email address of the account you want to use
Click Next
Enter your Microsoft Account password
Enter the current Local account password used to sign into the account
Click Next and you should be signed in with the new account.
