Hi
We used to show our Maine Coons and you can start from about 13/14 weeks. Breeders can tell from a very young age if a kitten is going to be what the judges want. When I first saw my Supreme Champ boy he was about 3/4 weeks old and between me and the breeder we felt he had a great chance on the show bench, and by his title above you can see she was right!!!
If you want to show the best person to ask about show hopes is the breeder you got the kitten from. If they've been a breeder for any length of time they will know and if it's good they should be happy to advise and encourage you to show. If you do well it gives their breeder prefix publicity and then they possibly get more business so it's in their interest that you do well. If they discourage you then I would think your kitten isn't show type and therefore they don't want a less than fantastic example of their kittens shown off.
Sorry if that seems a bit off bu thats what some breeders are like. It was stuck up snooty ones that made me stop showing, I couldn't be bother with the back stabbing of others doing well.
cheers