Depends on the person. I bought my son an OCR2 at 14 and although the reach was a bit long, it fits him nicely at 20. My father, on the other hand, didn't have his last major growth spurt till later. I built a bike at about 14 and didn't need another till I was at university.

At 14, you don't get too bent about bike fit knowing you'll probably need a larger bike later. On the other hand, with the amount of genuinely hard work a teenager will inflict on a bike, it doesn't really matter. Err slightly too large rather than too small and accept that a new bike will be bought in a few years time.

Besides, if old buggas like me keep buying and building bikes for no good reason, why discourage a lad coz he grew out of his.