For me it started at about age 14. A friend was front man boy in a high school band and they were short of a bass player. I played keys at the time, but he lent me his bass, and overnight I learned "Teenage Kicks". We rehearsed the next day and I was in.
I still have the enthusiasm. It waxes and wanes though. I sing and play a bunch of other instruments. But it seems there's a shortage of bass players, especially upright players, so bands tend to want me to play bass. Also, since picking up the microphone I'm not content being in the shadows at the back, I like being up front. Lately I've had my own home studio though, so I multi track more than play in bands these days.
My musical tastes moved away from pop/rock into classical then jazz then country, early music and most recently folk.
I've no idea what that very first bass I played was. It was a four string, black gloss body with white scratch plate. My main bass now is an unbranded laminate upright that's had a fair bit of work done, and set up "just so".