"[...] and the most useful skill, in my opinion, is being able to play by ear [...] "

Oh, yes. If they start trying to do this, they will probably hit a lot of wrong notes. This will diminish in time. My experience is that it seems to be more a matter of practice and intuition than learning scales, arpeggios and the names of the notes on the fingerboard, although I do practice these other things. I still hit far too many wrong notes, but I feel that I'm getting there.

And it's not the years one has played an instrument, it's the hours.