Probably need to define athletic performance. Are we talking about being a competitive athlete into our 30's and beyond, or living an athletic lifestyle and maintaining our level of fitness.
Constantina Dita Tomescu won the women's marathon in Beijing at age 38.
Al Oerter won a Gold Medal in the discus at age 32, a sport that requires both strength and technique. He set his PR at age 43, but the competition had improved over the previous 10 years and he was no longer a serious contender in international competition.
Bernard Lagat ran the mile in 3:47 at age 25, and 3:51 at age 35 (last year). He is still a competitive force in middle distance running.
Steve Scott ran 136 sub 4 minute miles in his career, the last one in his late 30's.
Elite athletes often live fairly one dimensional lives. They train, eat, sleep, and compete. With age, most broaden their interests and competition no longer becomes the central focus in life. Even then, many can continue to perform at high levels of performance due to improved knowledge and technique.
With the aging process we slowly deteriorate, but how quickly depends on how we live our lives each day.
So can we as backpackers (or other sports) continue to perform at a fairly high level? Obviously the answer is yes, as previous posters have provided examples. And how long can we do it? I still do 20 plus mile days in my 60's. But my lighter gear and experience have changed how I hike. The most I have done since turning 60 was 25 miles. But this wasn't a mileage goal, it was how far I needed to walk to get to my next water source and was planned. I am sure I can do more if I want or need to, but mileage in and of itself is not important... which begs the question, "Why?" I just want to walk, not meet any performance standard -- this answers the mental capacity question to "get there". All of this requires a fairly active lifestyle, and sometimes "day to day living" gets in the way of the lifestyle. So how long can I continue doing this without slowing down? I am not sure. If I stay healthy, maybe another 10 years, maybe less, maybe more.
I hike further than I did in my 20's and 30's. Part of this was due to heavier gear, and mentally 20 miles per day was a good days walk. I am probably less tired now at the end of a day's hiking than I was 30 or 40 years ago. I can still hike a 10,000 foot elevation gain in a single day, but not as fast as before, so I am thrilled with that. I cannot move at high sustained speeds as I once did, but can maintain a good average over a day -- something younger hikers don't seem to do; you know the hare vs the tortise -- which answers the pace question. But I focus on all the other aspects of the trip, and the mileage just follows. If I find some other diversion on a walk, then I may stop at 10 or even 5 miles.
As to the question about "other thoughts/concerns" -- An interesting thing about this is that when we are in our 20's or 30's we begin to wonder if we can continue at the same level, as Leslie has mentioned. When I turned 60, I started to think about my mortality. When you hit the 6th decade, your friends start to die off. And you realize you are running out of time, and unfortunately you don't know when the clock will stop ticking. Some of my friends have "thrown in the towel," and are waiting to die. I am simply continuing to live, without worrying about that final day. So I continue to hike (along with other physical activities - but backpacking is my passion), and age has not yet limited my hikes at all. So looking back, I would say just get out there and do want you enjoy for the pure pleasure of doing it. Don't worry about how fast or how long you can do it; but rather how much of your time is spent doing the things you love in life. And the more things in life you learn to love, the greater the journey.