Grew my first one when I was slightly younger than you are now. In the RN the official way to do it was to put in a request "To discontinue shaving". You then had three weeks, after which time your beard was examined. They did this by placing a comb in it, if the comb held you got to keep it, if not you were ordered to "Resume shaving". Once the beard had been sanctioned you could then trim it and shave the neck line.

I got round it by starting it off at the beginning of two weeks leave before going on Operations & Weapons Courses, this involved travelling round various shore establishments for different courses that normally lasted about a month. The first establishment assumed that I had gone through the official rigmarol in my last ship, and I we were never around long enough for them to check up.