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).

22 years old here, so not entirely adult-brained yet. I am in the process (well, while recovering from a surgery I'm taking a break, but...) of learning to play piano myself. It's slow going but pretty fun. If you have prior musical experience (for instance, in choir) it will be much easier. I've mostly been having difficulty finding appropriate (and interesting) sheet music myself. I mostly have been playing video game tunes, and many of those aren't particularly easy for beginners.