Originally Posted by Carbonfiberboy

No, no. He meant you reach your peak, whatever that is. Maybe that's a 10 minute mile. It matters in a way where you start. If you start at your peak, it's downhill from there. But for those untrained or only moderately trained, that 6-8 year span seems about right. There's another caveat: one also goes downhill with age. Obviously pros age out in their mid 30s no matter how they train. We see really good riders in the Under 23 group, and they've been training since they were kids. I started training in my early 50s and peaked at about 60, held it OK until about 63 and then it's been downhill ever since even though I've continued to train. My guess is that happens to pretty much everyone, no matter their age. Hence age groupers.

So your example is incorrect except for a tiny subset combining age and talent. If you can run a mile in 4:10 and you're over say 25, that's it. A person who runs a mile in 6:00 has a decent change of hitting 5:00, depending on age and training state and also of course, talent.