It's never too late to start. My dance teacher for four years, one of the greatest dancers I've ever known, didn't start dancing until she was sixteen. You may feel a bit left out because the other girls your age already have pointe shoes and are on an intermediate or advanced level, but that's no reason to be discouraged. Pointework itself is not too difficult once you've gotten up the strength for it.
You are right on the ball about the amount of energy it takes for ballet--it can be physically exhausting, especially once you break into the higher-difficulty classes. While most don't consider it so, ballet is almost more of a sport than an art form until you get it up on a stage--it can be taxing. And it takes buttloads of work to be able to do what the professionals do--those girls are dancing their asses off for five or six hours a day for five to seven days a week. It doesn't take that to get to that level, but that's what they do. Ballet is their life, they eat, sleep, and breathe ballet, and they do very little else.
As for the size thing, that just doesn't hold water to me. I'm 5'4", medium-boned, short-legged and long-torsoed, curvy size twelve, and I danced for ten years, the latter two of which were spent at this size. I knew girls of all sizes in my company. It doesn't matter what size you are; what matters is how badly you want it. Anyone can be a ballerina, at any size and at any age.
And I definitely have to agree with Antlers: it really does make you feel incredibly beautiful to dance. I didn't have the greatest self-confidence growing up, but as soon as that music started playing, as soon as I was in the middle of a grand jete across the floor, I felt incredible. There's not a day that goes by that I don't miss it, and there's not a time I see a dancer on stage that I don't wish was me. I had to quit because of joint disease, but the pull of ballet is lifelong.