I’ve heard learning music is a bit like learning languages. You can become proficient even late in life, with effort. However, for the young, the brain is literally still arranging itself, and if given intense exposure (be to language or music, preferably both), will achieve not only proficiency but fluency. Young people who are raised bi- or multi-lingual have the ability to think in more than one toungue in a way that, for an adult, may be physically impossible. I think this difference (which has a neurophysiological basis) is will nearly always demarc the line between “competancy” and “virtuosity”.
Of course, there are always rare exceptions to the rule, but such individuals who can achieve virtuosity starting late in life (meaning past their teens or twenties, probably) are rare indeed, and could be classified as geniuses.
So, yes, you can learn to play an instrument, and be greatly rewarded, as an adult; but it will not be the same for you as it is for a child who was raised on musical instruction. It’s not simply a matter of being a late starter: Starting late means you learn it differently, and that experience affects you differently. It’s still very worthwhile (as I can personally attest to, as I am now trying to get better at keys through self-instruction). But I must admit, I wish I knew better when I was young. I had the opportunity to learn to play piano, but was a lot more interested in building tree forts, collecting rocks, and finding new and creative ways to destroy those little green plastic army men. I wouldn’t have forgone my other hobbies, I just wish I’d made room for something edifying. I’m sure it would have done my brain good. Now, later it life, it’s so much harder. Worth it! But, alas, harder.