I work for a Mini dealership and ive taken people out before who have literally just said thanks after thrashing the balls off a cooper S and walked away. Problem is we cant be too careful because if we throw away business then we get a mouthful from the gaffer! So we generally dont refuse test drives unless its blatantly obvious they aint interested, for example a 17 year old wanting a test drive in a GP or something. So no reason why anyone should refuse you if you mean business. Ask if its company policy for an age restriction on test drives, if they say yes, then ask to see it and if theyre lyeing then you got one up on them, if not then fair does go to another dealership.