My meaningless opinion: if you know how to read, someone taught you. Either a person, or a book. If you can navigate a chord chart, either someone showed you or it was explained through reading about it. No one just picks up a sheet of music or a chart and knows what to do automatically. They were shown the ropes and respected their advice enough to take it. No ensemble would tolerate someone who just walks in, picks up an instrument, and starts blowing; they would stop, and show you what's up, assuming you're still invited to play. You don't learn as you go so much as you learn, and then you go; at least, not from scratch.
In that way, I think that very, very few musicians actually are completely self-taught. Eventually, I'm sure someone said to you, "hey, kid, I hear you're not quite getting this thing that's going on, let me show you how to approach it." I'm not assuming anything and maybe I'm completely wrong, but I'd bet that happened at least once or twice, especially knowing the jazz community I grew up in - its a dying art form and if no one shows it to the kids coming up, it'll be gone.
Now, I knew some punk rock kids that really, truly were self-taught. They didn't know how to tune up, what any knobs did on their amps, what any note was called, completely forget the concept of charting a tune; maybe possibly tab, but these kids weren't interested in playing anything at all unless it was something they came up with on their own. That's truly self-taught IMHO
I know a lot of other guys, who consider themselves self-taught, but they've had lessons here and there, took advice from other cats from time to time, read advice columns from respected players, watch instructional videos, stuff like that. "Never taken formal lessons" is more like what I'd call them. They are great players, but they have a lot of shortcomings from never studying with a teacher that I see but they don't see in themselves - stuff like timing issues, knowing theory, music reading issues, other various issues that they just don't get, but they could if they knew how to figure it out.
I'd put Jaco in the category of "never taken formal lessons" but "knew how to figure it out." But not "self-taught," since people taught him how to speak the language, and that's really what it's all about to be a player as diverse as he was. What I'm saying, is that people did indeed teach him.
FYI I never addressed "self-taught" as necessarily demeaning, its just that I have come across way more people who are self-professed self-taught that couldn't swing their way off a teeter totter than could.