Bought my brand new Impreza STi in '07 when I was 24... end of the day, if you want a car, go and look at the car and test drive it. If they choose to mug you off as a timewaster without giving you a chance, then it'll ultimately be their loss.
I'd say to take a bit of time looking around the car he shows you, ask a few relevant questions, before enquiring when it would be suitable to go for a test drive and he should take you seriously. But asking about stuff like its 0-60 and asking how fast it goes flat out might get you treated like a tool out for a joy ride.
Cheers,
Grant