I learned piano as a kid (10+ years training) but started on the mandolin and banjo as an adult (shortly before turning 30). I started learning mandolin on my own, using online tutorials and other teaching materials. In retrospect I tried to advance too quickly and didn't drill enough on the basics, reaching a local maximum and getting frustrated about my inability to improve further. Even a small amount of early interactive tutorial would have helped me a great deal in correcting problems of hand positioning before they became habitual.

I also found that I was terrible at playing with others even though I felt like I was doing just fine playing along with online tracks - lack of awareness was a significant hurdle. I started to learn the banjo a year later and took lessons from the start - learned some things at the very start that improved my mandolin playing even though the mechanics are very different (at least on the "picking" hand).