I'm 49, and I don't think that's terribly different form 50. I feel fitter and better than I ever have in my life-seriously! (And I was a college athlete...) I have more energy, I eat better, I sleep better, I handle stress better...All because I'm training for the tri.

I also train smarter than I did when I was younger, listen to my body better, know more about nutrition and health...

I think that if you do it right (The only concession I've made for age is that I have taken a year to get ready for my first tri so that I won't succumb to "too much too soon," but that might not be necessary for everyone), there's no reason to think that training is going to do anything but improve your life. You can do more than you ever dreamed you were capable of doing--but you'll never know that if you let fear of trying get in the way!

Go for it. Use this forum and others for encouragement. I'm on another one for triathlete through runnersworld.com (I started out as a runner), and there's another called beginnertriathlete.com. You'll find lots of people who are in this terrific time of their lives and loving every minute of it. In fact, I read an essay recently in Newsweek that was about the fact that there are so many Masters age people doing marathons--it could just as easily be about triathlons...

You'll love it!