There are already several categories of licence with age restrictions for motorcycles depending on power output. Since cars have a far greater potential for risk to other road users, particularly as they get bigger and heavier, let alone if they have a high power/weight ratio, it shouldn't be so surprising that it's a consideration. I can just imagine the outcry if you couldn't apply for a licence for a top of the range sports car until you were 24, as is the case for some comparable motorcycles.