l have a mens size 9 size foot. Is a snownboard with a 24 cm width wide enough?

Recommended Width Based on Boot Size

8 or Smaller
24cm or less
(Narrow)

8.5 - 10.5
24 - 26cm
(Regular)

10.5 or Larger
26cm or More
(Wide)

So regular to narrow. I'd say regular.

Well, are you young(will you keep on growing).

You want part of your toes to hang over the edge of the board, but just a slight, slight bit. It really depends on what type of riding you do. If you specifically do tricks, you want a board closer to the size of your foot. Basically, any board that is 5-6cm wider than your foot will do, that way when you factor in your boots & bindings, your foot is just at the edge.

That is not accurate at all newman, I've known plenty of people with size 12 is riding boards with only a 25.5 waist width and having no problem with toe drag, But to the main question yes a size 9 foot is perfectly fine on a 24 waist width.