I'm late 50s, and what I've noticed like you see in these posts is everyone ages somewhat differently. It's hard to predict. You can try to maintain superior shape as you age, and you will certainly be far better off than people who don't exercise at all, that's for sure. But your ability to maintain hard training and/or post good times in races may fall off faster (or slower) than people who were of similar ability to you when you were both younger. Injury susceptibility is something else that varies regardless of how fast or slow you were/are.
Part of the variance in aging slowdown between people is probably due to genetics, just like talent is. But life or career circumstances can intervene and change your motivations and goals which makes it hard to tease apart. And motivation can wane on its own, too, but that in part has physical roots. Just how much you probably never really know, which is another confounding variable in the mix. Here's how things have gone for me.
High School: 4:34 mile, 9:40 two-mile, 60-70 miles/week off-season training (mostly "LSD" with maybe a threshold run per week), 40 in-season when doing 2-3 hard interval workouts and speed sessions per week plus racing.
College: Ran XC season as a frosh and made last man on varsity, but dropped out after the season because the coach and training program were injuring or otherwise burning out too many runners.
Rest of College: Kept running because I loved it, probably 50 miles/week for the most part, but on occasion more like 60-70 when logging a bi-weekly longer run such as 12, 14, 16, 18, 20 miles. For me this period was the peak. Got in better overall aerobic shape than I had been previously even though not training competitively. Had I added intervals and raced, I likely could have improved on previous race efforts.
Post-College: Kept running for another year or two but then started my own business. Had a tiger by the tail, got completely swallowed up working 70+ hours/week, and dropped all running. I "woke up" 10 years later with health suffering in a big way from the stress, poor diet, constant sugar/caffeine to keep going, lack of sleep.
Mid-30s: Started running again and cleaned up the diet, sleep, stress issues to begin slowly getting my health back, then moved into training seriously. Figured I would be able to get back to at least some semblance of my previous shape, maybe to the 80-90% level. But whether it was the 10 years of sedentary inactivity, constant stress, and the associated health knocks running the business that had taken a toll, or simple age, I wasn't able to get back to where I thought I should have been.
Made it up to 40 miles/week a few times, but overall could sustain only about 30-35. Whether in ideal circumstances I could have rearranged the rest of my life to eke out whatever additional mileage might have been possible is moot. Aside from that, my competitive fire was still there, could still get in 2-3 hard workouts/week (mostly threshold sessions and 100m or 200m strides/repeats, plus a few intervals), and could still put forth 100% exertion when desired.
PRs at age 35/36 or so (rounded): 17:40 for 5K, 23:15 4M, 37:10 for 10K in road races. To compare with my PRs in high school, the 5K at age 36 would equate on the WAVA age-grading charts to about a 5:11 mile or 11:02 two-mile for age 17. Nowhere even close to the 4:34/9:40 from h.s., not even on the same planet. The people I knew of similar ability who had kept running all along had not slowed down as much race-wise.
Late-30s: Could still put out 100% effort but recovery ability started going down the tubes. (Decline in recovery in late 30s is the most common refrain I hear.) Never had very good recovery even as a youngster and had to do more "LSD" even then to maintain any sort of mileage, but now it was getting frustrating. My recovery days were such slow slogs, eventually I said what's the point, started running every other day instead to be fresher for the workouts that really counted, mileage dropped to 20-25 per week. 2 of the 3 days/week I did run were still pretty hard workouts, however. By the time I hit 38 or 39, though, I threw in the towel as far as competing. Still enjoyed training, but putting out a supreme effort in a race was becoming just too excruciating even when in good shape.
Age 40/early 40s: Recovery ability continued to decline, now only had the desire for 95% effort when any max-type effort attempted. But still ran with an aim toward being in the best shape I could be (without killing myself) just as part of my personal "fitness code of honor," and enjoyed training hard on the days I felt like it -- maybe once/week.
Late 40s: Had been away from the pain of racing long enough I got the bug to compete again on the local age-group road-race circuit. Put in about 9 months of training (3x/week every other day, every 2nd workout a hard or medium-hard one, 20-25 miles/week) gradually ramping it up, but eventually needed a month of downtime running very easy to recharge my batteries before hitting it again. But when starting the next phase of training, to my chagrin I found I no longer wanted to face the 1 or 2 hard workout days I had planned each week. Could I have? Yes. But mentally I was having a hard time facing the level of workouts anymore it would have taken, so I gave up the "mini-comeback" dream.
50+: Now mostly run about every 3rd day (usually need 2 days between runs to recover, even after relatively easy runs), sometimes every 2nd day, occasionally every 4th day, about 10-12 miles week, 3-4 miles per run, once in a while continuing an extra mile or two. Every 3rd or 4th run will be a medium-hard threshold-effort run or a couple of medium-hard repeat miles or half-miles, which I still have an appetite for, and it maintains better fitness/health than all easy runs.
But any time I have attempted what in the past would have been some kind of workout you might do as part of getting race-ready, whether it be a sustained hard threshold or AT-type of run, or a medium-hard long run, the toll is too great. My body will be strung out most of that night interfering with sleep trying to repair the damage, I'm dead meat the next day, my health suffers somewhat, and washed-out enough my productiveness and focus at work suffers. Also since my mid to late 40s, I intermittently fight IT band syndrome if I go too much beyond a 4-mile run, but don't usually want to that often, so it's not much of an issue. I could probably fix this with a very concerted sustained stretching program, and maybe will eventually, but not critical given the limited mileage now.
I no longer try to be a "hero" on a workout -- even if feeling good, I stay back from the brink a certain amount. The gauge now is running for fitness and for health. Any workout that knocks me for too much of a loop that night or the next day tells me I need to back off.
A few years ago, I unexpectedly became interested in bodyweight strength training by chance circumstances. It's turned out to be a way to excel fitness/athletic-wise in a different area I'd never attempted before. For the time being I have a bit more motivation for this than running but I like the balance of both. I've enjoyed gaining about 10 lbs of muscle, though most people don't really notice other than the fact I no longer look underfed. The extra weight has slowed my running down even more, but the balance of strength and endurance I find is better for my well-being and overall functionality at this age.
Recently an old teammate from high school, who was a more talented miler/two-miler than me then, began running again for the first time since college, and we try to get in one run a week together. As slow as I am now, I'm significantly better off than him. He's having difficulties rehabbing a bum knee sustained playing in the backyard with his kids a few years ago. But at least he can run a few miles (carefully) a couple times a week now and enjoys it. An easy pace for me now is 9:30/mile (threshold pace more like 8:00-8:30/mile), whereas for him easy is 10:30-11:00/mile. But if you can run and enjoy it without fighting injuries too much as you age, it's all good.