alt Hacker NewsThis is a great way of phrasing it -- ROI. My mom is a violin teacher and when I asked her if I could learn, she said "no, you don't have the hands for it". She recommended something like guitar would be better for me.
Not everyone can do everything, nor is everything a good return on investment. If you tell people they can do whatever they want, you are effectively wasting their time. Better to give them some useful advice, e.g. your fingers are better for the guitar, rather than insisting everyone can do everything.
Everyone can do much better than they think though.
My daughter's violin teacher refused to teach any adult because according to him if you're too old you're a lost cause ;) I agree an adult is likely not going to be the world's top violinist but I'm also sure that with enough work you can make vast improvements.
On both the guitar and on the violin you are not magically going to get to your max potential. It requires a lot of work. On the guitar you can get to a point where you can play simple songs relatively quickly. On the violin you do sound awful for much longer so it does require a lot more work to get to a reasonable level. Whether that's worth your time or not depends on you.