Learning to play piano later in life. I'll never be close to being professional but I've spent enough time and effort in practice to become proficient enough to confidently play in front of others. My only regret is why I didn't start earlier.
Until I started learning something as hard as the guitar, everything I'd had to learn in the past had come pretty easily to me, so I didn't need to study hard in school, etc, but that meant that I just passed on the really hard things (like advanced math). I think I lost out on the life-lesson about how effective diligent practice is at improving skills. I might have tackled even harder things earlier in my life.
I think very young people might have an advantage in how quickly they gain the required dexterity. However I've usually associated a young person's ability to learn the guitar quickly with the fact that most young people have a lot of free time on their hands and they just spend a lot more time practising than a busy adult.
Unless you've got an explicit ailment (eg arthritis) I don't think age is a barrier. "Working man's hands" might slow you down a bit at the start. I guess I'm lucky to have "programmers hands" :)
I had been a musician, playing clarinet and sax, in high school and college so I knew how to read music and knew a bit of music theory. I decided one day that I wanted to learn. I bought an inexpensive Yamaha keyboard and a beginner level book and started working through it. It took me about a year but I was playing songs using both hands and with lots of mistakes.
At that point, I decided I liked it and wanted to continue so I bought a new upright piano and continued learning from other books and finding popular and jazz music I wanted to learn and just dove in. I'm probably a piano teacher's worst nightmare but I don't care. I love to play and I would love to get better but, at this point, I just can't devote the additional time at the keyboard needed to do that. And that's OK by me.