I dont agree that just because something has "always been done that way" makes it right or legal.
However, like insurance companies can charge higher premiums for younger drivers, it seems that car rental companies can too. An insurance company can consider lack of driving experience in determining rates. It is only unlawful to consider the following factors in determining the premium rates to be charged for an automobile insurance policy: race, color, creed, religion, national origin, ancestry, location of residence in this State, economic status, income level.
read this old article from the New York TImes: