I think if you are going to issue a license to some, excluding others, you must have some non-arbitrary standard for why and who you are going to exclude, that is based in some sort of discernible governmental interest.
For example, people under 16 cant get a licence to drive a car, not because we don't like people under 16, but because people under 16 are scientifically less likely to have the judgement required to safely operate a dangerous instrument. You see, there is a discernible government interest in protecting the health and safety of other people on the roads. This limitation is based in a standard that promotes the governmental interest of protecting the health and physical safety of the citizenry.