51 in 18 days, I was 28 when I bought my first mountain bike, a beautiful blue/white speckle Ridgeback 602, from Bicycle Doctor in Rusholme, Manchester.
They sized me up (correctly) for a 16 inch, but when I got it home I thought it looked too small. I took it back and demanded an 18 inch, they tried to educate me about bike geometry, but I insisted, and eventually they gracefully obliged.