I have often thought about how good I could have been, had I started earlier. I never really exercised until I was 33. I am still improving year over year past age 50, so I clearly have not ever hit my top performance.
This year I set two objective PR's. The first was the Boston marathon, the second IM Arizona. I ran a marathon that was age-equivalent to 3:01. That is not super great but it is not bad either. Especially considering that I am a pretty poor runner. I also think I can run faster, and a 3:10-15 should be do-able. That would be age equivalent to a 2:50 or so.
I rode the Ironman at 215W average for 112 miles. That too is pretty good. It would be even better if I were lighter or more aero. Neither my weight nor my position is optimal. I am pretty heavy compared to an elite cyclist and at least 1o lbs heavier than I was in my 30's. As for aero: I am not even close to being aero when you look at the pictures. I sit straight up, and even in aero position I am high. I have this picture of myself riding with a pro triathlete. It was taken at IM Canada in 2006. I sit up and he is flat. You can judge for yourself.