I don't remember exactly how long it was before my first show, but my first show was only walk/trot (shortly after that I learned to canter).
Switched barns and trainers, a year or two later I did flat classes (w/t/c), crossrails, and verticles. Those were just schooling shows, then I did one event... and now... a few years later again, I am getting back into eventing.
I agree that you should establish a good foundation first, then consider showing. Schooling shows are a lot of fun if you can find some near you! While you should have a certain amount of experience prior to entering the show ring, you don't have to be able to jump 3 feet perfectly before you go. Flat classes are fun, and any experience you can get will help out down the road when you move up levels and difficulty.
I didn't show on a regular basis, but my pony and I almost always placed at the schooling shows. My horse has only shown I believe 3 times... twice at the schooling shows and one event.
It is a hard mindset at first, but go with the intent to do your best, for your horse to do his/her best, and to have fun. Show them what you've got and hopefully the ribbons follow. Just because you don't place doesn't mean you didn't perform well..... it just means it is time to go home, practice some more, and return next show to kick some butt!