The following are roughly the peak ages for various distances:
Sprinter- Age 26
Miler- Age 29
Marathoner- Age 32
Obviously, there are some exceptions to the rule. Carl Lewis ran his 100 meter PR, which was a world record at the time, at the age of 30. He was still a world class sprinter ranked in the top 10 in the world up through age 35. Meb Keflezghi set his marathon PR with a victory at Boston just a few days shy of his 39th birthday. Even at 41, he was still good enough to make the U.S. Olympic team. Bernard Lagat nearly won an Olympic medal at the same age in the 5000 meters.
Realistically, the 19:09 5K that I ran 10 days shy of 34, was probably my last personal record but I've met some people who honestly think that my age is the reason that my performances have fallen off a cliff in the last 2 years. Some might blame past over-training. I do not. Even at my peak, I never averaged more than 42 miles per week over the course of an entire year. Yes, I did over-reach at times by forcing too many quality sessions in a single week but I almost always took a rest day when I needed it and took cut back weeks periodically when I had a race coming up soon. If I had it to do over again, the only thing I would have done differently was race more frequently when I knew that I was hot. I believe I had a sub-19 5K in me on a few occasions as well as a sub-5:15 Mile. I never broke 40 for 10 K in a race setting but the time trial counted the same to me. I did get the one that I wanted most, which was the sub-90 half marathon.
Now to address the main point of this post:
I did some research on the median finish times in every age group at the last 2 Mercedes half marathons among the men. I found some intriguing results that are likely to surprise my readers. Teenagers were the fastest age group followed by the 20-24 division but they also had relatively few finishers. Starting at the 25-29 age group (when I debuted), things got interesting. Listed below are the median finish times and number of finishers from the M25-29 through M50-54 averaged between 2015 and 2016 to guard against any anomalies:
25-29- 2:05:12 (145)-9:33 pace
30-34- 2:03:59 (176)-9:27 pace
35-39- 2:01:41 (215)-9:17 pace
40-44- 2:05:30 (214)-9:34 pace
45-49- 2:07:39 (187)-9:44 pace
50-54- 2:10:33 (170)-9:57 pace
If I were to guess the training of mid-pack finishers, I would say it's only "semi-serious" with an average weekly volume around 20 miles with ups and downs throughout the year and a build up which includes several double digit runs as race day approaches. Of course, there are exceptions based on the amount of natural talent.
In both 2015 and 2016, the 35-39 division was the fastest among the age groups listed so it's unlikely to be an anomaly. Why is that? I'd chalk it up to more experience racing the distance. In my first 2 half marathons, I really didn't know what I was doing. In my first, I went out too fast and had to walk-jog the last 5K. My second time, I did the opposite and finished strong but left time on the course. Also, runners in their late 30s are probably a bit more serious about training and less likely to jump in unprepared. In terms of raw physical ability, I'd say those in their late 20s do have an edge over the late 30s but not a significant one.
As for the age-related decline, a 40 second drop off in pace between the 35-39 and the 50-54 seems like a lot but if you average it out over 15 years, it amounts to an average annual decline of 2.66 seconds per mile, which should not even be noticeable from one year to the next. Expressed another way, it's just under one half of one percent per year. Being 35 or even 45 or 50 is NOT a death sentence. There is most definitely something physically wrong with you if you fall off a cliff at that age. I will not quit until I find the smoking gun.
As for the participation rates, it seems like that tends to rise from age 25 before a peak around age 40. Similar to the performance, the rate of decline is slow up through the early 50s. Things got interesting in the 55-59 age group. While the median finish time did not decline significantly, the number of finishers fell by nearly 40%. I would guess the reason for that is top performers are more likely to stay with the sport than those in the back of the pack. Starting with the 60-64 division, the attrition rate is about 50% for each subsequent division and performances decline sharply among those who stay involved. Male finishers over age 75 and females over 70 are very rare.