I think it depends on the dealers insurance and the salesman you are dealing with. When I was 18 I used work in a jag dealership and had to drive Jag XKRS's, XJRS (v12 supercharged) and limited production TWR twin turbo converted XJ saloons, some of them were in excess of 600bhp.
Basicly it's up to the sales person you are dealing with, if they think you are just turning up to mess them around, waste fuel and get a sales car dirty for no good reason then, naturally, they'll tell you to do one.
My dad's is in his 50's and has been refused test drives in certain cars for one reason or another, so I dont think it's always down to age. When he went to buy his SLK 320 I went with him and the sales guy even offered me a test drive of it to secure the sale, basicly he knew I'd convince my dad to get it after taking it out for a spin, making his job easier.