^ That is a myth, and will always be a myth.
The longest I've ever let my hair grow is a week and a half, and it was really uncomfortable and looked ***** - my beard's pretty sparse anyway. I think what the OP is really getting at is that some guys seem to effortlessly sprout thick bushes whenever they feel like it, whereas others can't - presumably the former have more testosterone or something.
Either way, not being able to grow a beard doesn't make you any less a man really... I mean, in some areas it's an advantage I suppose (ie. Armed Forces - I hate shaving at 5am in minus temperatures in the middle of a training exercise on Salisbury Plain... pure HELL).