Pretending that we don't age would be silly. I don't need a stopwatch for that. I've got a mirror. And, at age 53 now, I've also got memories of running in my teens, and 20s, and 30s, and 40s. And of course it changes over the years.
It used to be much easier, now not so much. Doesn't matter what element of training or racing we're talking about. Every component of our running bodies weakens as we age: We lose muscle (mostly fast-twitch), our connective tissue stiffens, our hormone levels drop, our nervous system slows down, our max heart rate drops (and the corresponding cardiac output), our VO2max gets lower, etc. etc.
When my shoelaces came untied mid-run in my 20s, I didn't have to find a bench or low wall to retie them. In my 20s, I didn't worry about pulling hamstrings and quadriceps attempting intervals without a running start.
But then again, age gave me a great gift: It removed the ability to train stupidly and hope to make a race start line; it forced me to become a better runner, even as I became a slower one. I soon learned that training as an aging runner is a No Mistake Zone: that I had to practice proper workouts (targeting all elements of running fitness--not just a glut of mileage and fast reps, with the hope that fitness and performance improvement would magically trickle down), proper recovery, targeted and thoughtful racing, etc.
And as I trained smart, I began to view racing not so much as a competition against the past--a race I could never win--but as an attempt to maximize my current potential. Victory has become maintaining a smart training routine and then celebrating with a race in which I run as fast as I can--at this age.
Look, here's the thing: As runners, we learn early on--unless we're Olympic champions and world record holders--to measure our performance against what we consider to be our own potential. To enjoy running as you age, do the exact same thing; just recognize that the "you" who existed at age 25 is not the you who will exist decades later.
If there's one thing masters runners are proving these days, it's that we don't have to slow down as much as we thought we were going to. When I was 21, I was amazed to discover that runners in their 40s could still run a 5 minute mile. At age 50, I averaged 5:01 for a 10K.
Don't be limited by what you cannot do as you age; be amazed by what's still possible.
Either way, you will age. It's your choice how you choose to deal with that--but remember that most of your life will be lived on the downslope of athletic performance. That doesn't mean it can't be a wild and exhilarating ride all the way.