When I was 13, I decided to learn an instrument - guitar or bass. Anything else (at the time) was out the question. I picked bass because a) I loved that low end rumble when I tried one and it just seemed much cooler! 10 years on I still believe I made the right choice. I started learning guitar when I was 17/18 but I'd still pick bass everytime.
It still makes me laugh as well when you ask a musician what they play and they say 'I'm a songwriter'. If you play an instrument and write your own riffs then you're a songwriter anyway! Redundant term IMO.