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.