Well, my username is basically my initials and the age I left college and started to thread warily into real life.
I suppose my username reflects my individuality in a world where long lists of statistics are the norm while also showing my unique outlook on life and on the problems it frequently throws up. As mentioned above, the number 19 as my age shows the real moment that I started to truly move on into adult life, leaving my childhood and teenage memories, good and bad, behind me. Thus, DJM19 was created.
...Alright, maybe it's not quite so deep-thinking, but it sounds good. The unglamorous truth is that it was the first username that came to my head.