When I started music, I had no idea I would be playing bass in orchestras across Canada. My love of music started with listening to records when I was about three and singing along with musical kid's shows.
I started piano lessons (Leila Fletcher method) when I was about 8. I didn't have a piano but a small, spinet Hammond organ. (It had three different sounds for the top keyboard, two for the bottom, one for the pedal, no drum machine or auto accomp, but it did have a vibrato and reverb option!)
After progressing as far as I could without a damper pedal, the piano teacher told my parents I either needed to get a piano or start organ lessons.
So, I took organ lessons for about 4 years through a local Kawai dealership. I really learned a lot about music theory and how the bass part works in accompanying various styles of music. The left foot played the bass pedals and the left hand played the chords on the off beats. (Boom, chuck, chuck for a waltz for example, and so on.)
Meanwhile in school, I grew up with the Kodaly system in grade school (the Dartmouth/Halifax Nova Scotia area being one of the first areas in North America to have it) so lots of singing and learning musical theory plus playing recorder. In grade 6, I started on tuba/baritone.
In grade 10, I taught myself guitar because my English teacher played Mother from Pink Floyd's The Wall in class as part of a lyric/poetry unit. The first thing I taught myself was The Trees by RUSH. I did this by buying the printed music, learning it on organ, then translating every note to the guitar. There were also chord symbols in the music, so I learned those and because of the chord theory taught in the organ lessons, voila, I was an (almost) instant guitar player.
I should say that most of the rock music I listened to was stuff with interesting bass lines and well arranged background accompaniment - Rush, Beatles, Police, Tears for Fears, etc.
Also, I joined Air Cadets when 13 and joined their band playing glockenspiel in the glock'n'drum band. I went to the summer band camps a few times and ended up being on staff and eventually became the 2nd in command. I arranged pieces, conducted the band, put together marching drum shows, organized parades, etc., etc., etc.
Back to school. In grade 12, our music teacher put together a jazz band and I played the bass lines on tuba. The music teacher said there was a bass guitar in the storage closet, I said OK and tried it. Thanks to being able to read bass clef from piano & tuba and the bass had the same strings as the bottom 4 strings of a guitar and doing bass parts on organ, I could apply all these to playing the bass guitar and presto, instant bass player.
Later that year, I was at a rehearsal in the city's music building, I was putting my tuba away and there was a double bass sitting at the back of the storage room. Hey, that's what Sting sometimes plays! I picked it up and tried it. It was physically harder to play than bass guitar but I could play it decently.
I found a teacher who used to be a student of Gary Karr and she gave me some lessons (I still have the Simandl book) and rented me a plywood Hofner.
Near the end of grade 12, the Nova Scotia Youth Orchestra got a new conductor and he wanted to have the hugest orchestra possible so if you could hold an instrument, you were in. I joined and went to their summer camp. We played all kinds of bombastic orchestral music. Stuff I had zero experience with (except listening to Hooked on Classics and loving movies with John Williams soundtracks) and was completely blown away by the music.
With the NSYO, I played Beethoven 5, Brahms 2, Shosti Festive Overture, part of The Planets, William Teel Overture, Adagio for Strings and so on. It made all those rock albums seem so immature in comparison.
I started university on tuba but quickly realized I played double bass after 6 months than I did tuba after 6 years, plus looking at future work prospects, bass seemed more lucrative (and I'm really rolling in it now!

), I re-auditioned on bass and started taking real lessons with Lena Turofsky of Symphony Nova Scotia.
She eventually told me if I want to get anywhere in music, I would need to go to Toronto to study with Tom Monohan so I did. After graduating there, I studied privately with Ed Tait, also of TSO. While attending UofT, I played with the National Youth Orchestra of Canada during the summers - more amazing music and travel and meeting new people. I was addicted and wanted more.
I was just starting to free-lance gig around Toronto when everything ground to a halt with repetitive strain injury. It's a long, sad story that I don't want to go into details here but I ended up selling all my stuff and moving to London, Ontario with my wife to retrain to be a music teacher. (What was I thinking that I could be a music teacher without using my hands?!?!)
Anyway, it turns out one of the top chiropractors in the world lives here and I got a grant from the Services for Students with Disabilities of University of Western Ontario to get adjustments 3 times/week. So, after 10 years of that and various exercises, I was pretty functional if I was careful with my hands.
I sang in a high quality church choir for a few years to do something musical (although I couldn't even listen to music for several years as it was too painful to do). Then about 10 years ago, the church decided to start a string orchestra and the person directing it hears I used to play bass. I was rather reluctant but i gave it a try.
He brought in a school instrument from his daughter's school for me to play. Fortunately, it was well set up and easy to play. It went OK. And singing in the choir really improved my tuning!
One week, I decided I would take the bass home and play the crap out of it to see how my arms would hold out. They held out OK so I thought I'd try to give bass playing a go again. Just then, the local bass luthier Peter Chandler, was peaking at making basses and he allowed be to rent-to-own a bass from him.
I rented that and 6 months later I did a recital playing Bruche's Kol Nidre and Rossini's Duet for Cello and Bass as the big pieces. A few months later, one of the regular extra basses of Orchestra London quits and I get invited to play with them. I became their regular 4th player in the Masterworks and Pops shows for several years until they shrank the orchestra.
Since then, I've played in many of Southwestern Ontario's orchestras and am busy most weekends with some kind of gig somewhere. Outside of playing bass, I supply teach in the local schools. When doing neither of those, I am hiking.
So, from reading the other replies and talking to other bassists, we all seem to have had some strange journey.

I wonder if other instrumentalists have the same experiences or if it is just common for bass players? Personally, I find it strange when I hear about a violinist who started at 3 years old and just kept playing violin until they get a position at 23 and are still at it 20 year later.
(Zheesh, I didn't realize this would become such a novel, sorry about that.)