Bekky is on the money, being under 25 and asking for a test drive of any high performance car (particularly from a dealer) is a nightmare.
But if your selling, and they believe they deserve a test drive, you deserve to at least know if they've brought a deposit.
I prefer to take them for a drive in the car, maybe even scare them a little (feel them out first) by showing them what your car can do, after all, you've got years of experience driving it compared to them.. then if they ask to have a drive ask them if they've brought a deposit with them and are they prepared to buy today if they like the car.
Explain that you've had lots of test pilots and telling them a few stories about previous test drive nightmares you've had might stop them from driving like an idiot once they do get behind the wheel.
I also like to say "Mate you can drive it as hard as you like once its yours, but I've treated this car with respect the whole time I've owned it."
If they dont agree with that tell them to take a walk.
If your buying from a dealer, go to the ATM, take out $1k.. then when you think theres a good chance you might buy the car if it drives well, take it out and wave it in their face, hand them your license and say "I hate screwing around let me drive it, if I like how it drives, and my mechanic say theres nothing seriously wrong with it, you've got a deal"