Just listen to your pony. When he's done and feels like he's being pushed too hard he'll definitely let you know! 19 is getting up there in years for a horse, but if you listen to him and take the time to keep him fit and pay special attention to his needs then I think you'll be alright. I rode a 26 y/o horse at the preliminary level (schooling intermediate) a couple years ago and he was still going strong! Heck, he ran his last two-star when he was 21!
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."