Technically I started off on Trombone (and I don't mean to blow my own Trombone, but I was fairly good) at 8 or so. Played that for a couple of years, then gave it up, as it was a bit taxing for me.
At about 13 I started playing Steel Pans, on the Cello pans (serves a very similar role to Bass Guitar actually...)
Got a Bass for my 14th birthday, and have been going since.
We went into the shop on the day, and the guy in there said something along the lines of "It might be worth learning guitar first", to which I shook my head and reaffirmed my low end-ness.
I'd have struggled with guitar really, my hands are rather big, and the string spacing on a guitar is just too small for me.