As stated in the AdSense Online Terms of Service, developers must be at least 18 years of age to participate in the AdMob programme.
If you're under 18 years old
If you are not yet 18, you may have a parent or guardian submit an application using their own Google Account. If their application is approved, all payments will be issued in this person’s name.
When you're over 18 years old
If your account was previously disapproved for being underage and you're now 18 years old, you don't need to create a new AdMob account if your account is still open. If your account has been deactivated due to inactivity, first reactivate your AdMob account. You can then contact us for support to continue using your original account.