Couldnt find this anywhere, so here it goes.
If you by accident created your Microsoft Account on your Windows Phone and didnt change the date of birth it will just choose the current day. This will make your phone go into “Child mode” or something like that and you wont be able to use Marketplace and so on.
You’re unable to change this on your phone.
Go to live.com –> Login with your account –> Account settings –> Edit personal settings –> Change birth date –> Save and confirm and then reboot your phone. Seems to work.
No need to hard reset your phone like a lot of people says.