When setting any kind of Minimum Age and/or Maximum Age limits on a Registration Type, 2 fields have to be enabled on your registration form for this to work: Age and Birthdate.  The Birthdate field verifies that the Age a Participant is entering is correct to prevent a Participant from entering in just an Age and bypassing the limits.  You can find steps to add the Birthdate field to a form in your event here.

*Note:

When there is an age requirement, the form will check the age used in Birth Date . Sphere verifies the participant's age based on the Year of their birth and the Year of the event under Event Information.

If the participant's birth Year exceeds the Age limit, register the user with a false date which qualifies the limit, then change their Birth Date on the contact record via Contacts>Individuals>Manage>Profile>Personal>Edit.