I'm 51. There hasn't really been an age where I feel old, it's just some days/events. Like when my neighbor's sons come home to visit with THEIR wives and kids and I can remember when those sons were 3 and 5 and playing in the yard like it was yesterday. I feel old then. Or when I realize I can barely do a quarter mile anymore at the pace I used to hold for 20 miles.
Never feel old when running, it's the main thing keeping me young (at heart). Still do about 25-35m a week, just like the last 15 years or so, 10 miler on Saturday or Sunday at whatever pace I feel like doing. Can still do a 5k around 20 min, give or take a few seconds, off no speed work at all. Well over 5 minutes off my best time, but no worries - nice to be putting in the miles at all when so many of my former teammates have been felled by injuries over the years and wish they could still do it.
Life is good, still waiting for the "old age" bear to jump on my back in a permanent kind of a way, he only swats me from time to time.