I recently learned something stupid about FAA licensing. Once you have a license higher than "student", you have to have a biennial flight review in order to fly an aircraft. If you don't have a current review, you cannot be the pilot in command of any aircraft.
A student pilot does not need a current flight review, just an endorsement from their instructor, in order to fly solo.
So, if you are a former 747 pilot with 10,000 hours in the cockpit, and you seek to add a glider or balloon rating, you are not allowed to solo these aircraft unless you have a current flight review. Since solo flight is a requirement before obtaining these ratings, you cannot get a rating in one of these aircraft without getting a flight review in an aircraft you are already rated for.
A 14 or 16-year-old kid with a few weeks of training is allowed, but the air transport pilot is not.