At what age do middle distance pro runners stop improving

Average peak age of performance? 25 or so, perhaps a bit older. When do times stop improving? Depends on the athlete but some have been able to sustain improvements all the way into their 30s. Nick Willis ran 3:32 at 21 while still in college at Michigan, but ran his PR of 3:29.66 at 32 and medaled in Rio at 33.

After 30 its not really possible to run a fast 800 as its mostly dependant on muscle mass and youtful nervous system.

As for 1500, you can run it all the way until you are 40.

Id say peak 800 is 26 and peak 1500 would be 31

generally peak performance for faster races peak in early 20s while peak performance for longer races are low to mid 30s. this is not true for every person, however. since 1500/3000 is more on the distance side of things, i'd have to say maybe mid to late 20s is the estimated range for peak performance.

One caveat here is that there’s probably a bit of a gradation in how 800 runners progress as they age depending on whether they’re leaning more speed or endurance. Symmonds, for example, leaned heavily towards the endurance side relative to other top 800 guys — I think his best open 400 was around 47.5, whereas there were other 1:42-43 guys who ran 45.x — and I’d imagine that type of runner peaks somewhat later than the ones whose peak form was built on sprint work. When speed starts to go it goes quickly.

World record for 35M in the 800 is 1:43 low. That’s a pretty fast 800 lol, and plenty over 30 have run 1:42

Basically after 26 you won’t see many improving their 1500 times. Peak age is 23-25. There are obviously outliers but most don’t improve