When I was either 12 or 13, I think 12.
Also, I understand the whole "safety" element of giving new shavers blunted (safety) razors to use, but geez ... I had patchy weirdness for the first few times, as a new razor is actually safer than a blunt one by virtue of its smooth and complete cutting ability.
When I was 16, I decided to let my beard grow down to nipple level, ala ZZ-Top. That was the year I discovered sports bars as a viable hangout. Before you get the wrong idea, I just wanted to go have a glass (or pitcher) of beer, and unwind a bit. There was no drunkenness involved.
Just before I turned 17, I decided to shave it off for senior pictures at high school.
Now I compromise and keep a goatee that's between 1/4" and 5" long, depending.
Long story short: late in my 12th year, hence my selection.