I have thought I'm 29 now and my body responds very differently (more negatively) to training to how it did when I was in my early 20s. What in Gods name will it be like in another 10,20 etc. years?
Hmmm... I'm 43 now and I'm in the best shape of my life, so I wouldn't worry.
The biggest difference is that my sessions must be short, sometimes very short, but intensity haven't decreased.
Plus, with age you are cooler with the girls.