It didn't take me long to teach myself, in my late 30s, to sight-read (only the top-line melody, mind) but I find playing-by-ear/mimicking my above-mentioned Chants much more difficult (I'd earlier simply found a way to sing/chant my verses without being able to read or write music). On the other hand, someone sat next to me at a folk-club, who can't sight-read, worked out the fingering for a folk-song literally within minutes of me singing it - I guess it's at least partly knowing the instrument very well, as you say, Carol.