I grew mine aged 21, basically as soon as I could, because otherwise I needed my passport to get into a pub. After 3 years of being old enough (UK drinking is legal from age 18) that seemed more than enough. I think what really pushed me over the edge was a bus driver offering me half fare (ie I looked 13 or less). Also people don't expect to hand over thousands in cash to an apparent juvenile, however august the bank counter he is standing behind.

Of course now, 32 years later, I keep it out of a combination of habit, laziness, and fear that the pubs might refuse me service again if I shaved it off.