If you asked if 19 is too late to become a pro, than the answer would be probably no because becoming pro takes a progress of at least 12 years. But as soon as you can be fit in just 1 year (even less if you already have something), 19 is actually a great age to start tennis. You will be 20 when you have a fit body and when i think about my friend's daddy that started body building at the age of 50 and had a great body, i think 20 is more than a wonderful deal.
I'm 23 now and have always been interested in sports. I played soccer, basketball, i have done bodybuilding for 2 years till i was 20. I always dreamed about a body like brad pitt till i was a teenage but i could never achieve that with the sports i have done because i have never achieved consistency. When i met tennis at 20, i felt like it was my sport. It has been 3 years now till i begin playing it and first time in my life, i see a whole six pack when i look in the mirror. Loosing fat has never been so easy, i can lose 8 lbs a months easily by just enjoying at court for 2 hours and cutting sugar. Gaining muscle is also easy, you burn a lot, so you eat a lot while making some weight training. That is a wonderful thing for me because i like eating. I feel energetic, youthful and happy, that is something you can't do with the damn treadmills. I think tennis is the best sport that you can do because it offers you interval cardio which is the best formula for losing fat. When you are playing basketball or soccer or you swim, you always need spend physical efford, when you stop, you are beaten. In tennis, you always have the rest when the ball is called out or when the set or game is finished. So you can play and enjoy tennis for 2 hours without the feel of dieing.
Your goal is also important. If you wonder if tennis itself can give you a body like Nadal, i say it's not possible as long as you have a right technique. But you can have this by just playing tennis and have a proper diet (which is necessary for all the sports);
If you combine tennis with some weight training routine again with a proper diet, you can get a body like that, even more if you like to;
Also i think a lot of people in this forum started tennis after they are 30. They do that because tennis is a sport that you can even play with and beat your grandchildren. Not to mention that it will give you a healthy, quality sportsmen's life which is priceless.