My one instructor only started riding after school, and she's 27 now, and she's an excellent rider. And anyway, she only went to her first show 2 years ago, not because she lacked ability before that, but because she couldn't have been bothered to go to a show if it wasn't for us asking her to.
Also, I'd say cantering and going over trot poles after only 5 months of riding is pretty darn good. Sometimes it takes ages for people to reach that stage, so be happy with the pace you are learning at. At the rate you are going, it shouldn't be too long before you can start showing.