I'm a doorman and I only check the IDs of people who are clearly over 50 years old
That's confusing to me? Why do you only check their ID if they are an older adult?
I'm not allowed to because all the people I exclude are balding
Choosing to only check the Id's of older people is discrimination and it is wrong. If your job is to verify the people that enter a building, you should do so for all people. You could lose your job if the older people figure out what you're doing and decide to tell management.
Haha, they wouldn't do anything, they aren't going to give a shit, they don't know they are all balding.
People most likely do know that they are balding, but that's not even the point. You shouldn't discriminate against only older people or else everyone will believe that you are ageist.