I think I've always had it. I remember having trouble understanding what the term meant when I first came across it, because I thought it was just part of how hearing works. (And sometimes the sound I hear is real-- I can hear CRTs, for example.)
The only time it has ever bothered me was when I had a bad sinus infection and both ears were blocked, reducing hearing. Then what I had wasn't really tinnitus, but my mind trying to fill in the missing sound, just as one will sometimes "see" things in complete darkness. I didn't really mind the sounds my head came up with, but it was a little surprising to discover my brain was capable of generating swing by the Count Basie Orchestra, complete with bass line.
