I assume that the '75 or under' rule is to comply with the vehicle's insurance policy.
Although the age discrimination legislation doesn't apply to people over 65 (or to voluntary posts), I can think of some people who might genuinely see this as age discrimination. I know a guy who's a lot closer to 90 than he is to 80 who still works full-time (and for long, unsocial hours with some heavy lifting involved) running a wine shop and off licence. His only assistant is his wife who is also old enough to argue that the notice is discriminatory.
Chris