I think the age is actually linked to the device, not the account. On android, I think you can log out, uninstall, reinstall, play the guest account and sign in with an older age, then logout of the guest account and sign back in with the Google account. On Apple, you might have to change something in Game Center.
Other options are just to start a new account, underage or otherwise. There's not a lot of progress after only a few months, and kids are pretty resilient.