We know Stefan was 17 years old when he turned vampire. But how old was Damon when Katherine turned him?

I'm not sure if they've ever said. I also imagine Damon being maybe early 20's? I'm not positive though.

he' is somewhere in his 20's.

Vampires don't age.

22

24! Their mom died when stefan was 11 and damon 18 , therefore there is a 7 year age difference between the brothers. So if stefan was 17 and damon died /got turned the same night he was 24 years old.