Interesting topic–one I like to ponder from time to time–and it can be looked at from a number of different angles.
Peak performance (your best years): Most have 10 to 15 years. start developing by age 14-15, improve for 7 to 10 years and if you can keep healthy an at it hold on at about the same level for 5 or 6 (maybe 10) years.
Late starters will follow a similar arc but the improvement curve is often shorter on the development side and not as high. When was the last time you heard of a sub 2:10 marathoner or 13:15 5K runner who started training at 25? But sometimes you do get 2:20, mid-low 14 in types who started late and peak in the 40s.
Indeed some of the best masters runners were late starters or early starters who took a long break (Pete Magill). A few have kept at it and were great at 25-30 and were still good-great at 50 or 55, e.g., Bill Rodgers, Joan Benoit Samuelson, Linda Somers Smith.
A very small minority can peak or almost peak at 40-42, but then things drop off fairly markedly. The level and number of competitors also really declines at 45 and beyond.
At 50 it was like I looked around and said, hey where'd everyone go? I now run 5K 2 to 2.5 minutes slower than 20 or 25 years ago. Back then I was just a decent local runner and didn't have to travel far to get pounded into the ground by good former college runners within 5 years either way of my age.
The other week I won my age group in an okay effort (17:35), but was 2.5 minutes ahead of the next guy. It's not just the pounding from running either; same thing is happening in XC skiing. I'm staying at about the same level as at 46-50 and the other guys are just falling back. I grew late, is that a factor?
There is still a big gap between some of the top guys in my age group (so far I think 4-5 guys in my age group have already posted mid-low 15s for 5K this year but there are very few between that and say 16:50.
Mileage wise, I do about the same I did through most of my 20s, but months of high mileage are not sustainable so I spend half my year skiing. But yes, can't do the quality. It takes 10 days to recover from a set of hard reps at 3K or 5K, so now it's more tempos, a little speed, and then use the races as the workouts.