Learned on my dad's '77 Stingray fretless with flats around late elementary/junior high age.
When I entered high school, I actually took music classes for the first time. So the Music Man was not exposed to the possibility of theft, loss, damage, etc., I took his '76 Musicmaster to high school and kept it there in a music locker for first semester of freshman year, when I was in beginning band. At this time, I insisted on stringing the Musicmaster up with flats, as that was what I was used to playing, due to playing/learning on the Music Man. Second semester of that year, I moved to jazz/stage band, and I was allowed to keep the Music Man locked in the band director's office. This continued for all of sophomore year and the first half of junior year.
Sophomore year, I learned upright and played it in the orchestra until the end of high school (a school bass - some German-made one the maker of which I don't remember).
Junior year, my dad decided he didn't want me keeping the Music Man at school any more, so about halfway through the semester (my b-day/X-mas), he got me a slightly used G&L L-2000 fretless for the crazy low price of $400 (it was only a few months old, in dead perfect condition).
Senior year, I switched to trombone in jazz band, and didn't play bass guitar at school any more (only upright in the orchestra).
That G&L was my only bass until 2007, when my dad died, and I inherited everything he owned, including the Music Man, the Musicmaster, and a Franken-P-Bass made of MIJ, MIA, and aftermarket parts (with the flats I had installed for him years earlier).
That all sat for about a year (I was almost entirely playing guitar at this time). Then I decided to join a band on bass because I missed it. I used the Franken-P for this band, and ever since, I haven't played the fretless basses but for a handful of times.
I joined another band in 2008, which started playing gigs in early 2009, and I left the other band behind. I played the Musicmaster in this band, with a pick (but with the same flats I had put on the bass back in the early '90's).
As the years in that group went on (we are still together), the Musicmaster was my main axe, with the P being played from time to time. (After a few years in this band, I ditched the pick and went back to what I was most comfortable with: my fingers.) But I picked up some other basses and messed around with them for a while: A Squier Jaguar short scale (modded with 1/4 Lb. pickups), a Squier Bronco (kept stock), and a no-name cheap-O short scale Japanese bass from the '70's (which I tuned to D standard using the bottom four strings from a 5-string set). All of those had rounds on them. I moved them along to new owners around 2013, because my collection of instruments was getting out of hand, and these were my least sentimentally precious and most redundant basses.
In 2014, I was hired by a "known" blues artist, for whom I had auditioned back in 2009. Back then, I got beat out by someone else. But when he quit, they remembered me, and asked me to re-audition, and I got it this time. My introduction to working with this band was two two-hour practices (without the artist present) to learn about 30 songs, and then directly in to a 30-day, 20-gig southeastern and Texas tour by van. I used the P for that, and a few extraneous local gigs with them after the tour was over. (This artist has since died.)
This year, I picked up an AV '64 J (my first ever Jazz Bass) and an AV '58 Precision (my first ever bass with a maple fretboard). I just got .110 La Bellas for both of these, so I can finally set them up the way I like and move them in to the rotation. I will probably be using the J primarily for a while.
In short, here is what I have now:
'76 Musicmaster, Olympic White
'77 Stingray, natural, with '80 Sabre fretless neck (almost never played, but kept because I learned on it)
'90's L-2000 fretless, ash, natural with ebony board (almost never played, but kept because it was my own first bass)
Franken-P, black ('90's MIJ body, MIA Fender replacement lacquer neck, Duncan pickup)
'13 American Vintage '64 J Bass, Olympic White (new to me, about to enter rotation)
'16 American Vintage '58 P Bass, blonde (new to me, about to enter rotation)
...and here is how I got to it:
1997: + L-2000
2007: + Stingray, + Musicmaster, + '62 RI Franken-P
2010: + no-name SS
2011: + Jag SS
2012: + Bronco
2013: – no-name SS, – Jag SS, – Bronco
2018: + '64 AV Jazz, + AV '58 Precision