Your 20's and 30's is prime time. Anyone who thinks they are slowing down in those years is because they are slackers. Of course this doesn't apply to elite athletes. I was in 95% in my 40's. When I hit my 50's I started slowing down. In my mid 50's was the big decline. Sure there are guys who are 55 running 18 minute 5k's. They were also capable of 15 minute 5k's in their early 20's.
What pisses me off is these old goats who juice and take all these compliments like they deserve it. They would look like crap without drugs and they know it. When the drugs start to effect their heart and prostate they will have an almost instantaneous decline when they have to stop using.