I still try to run competitively in my age group. There's not enough data to work out a formula for successful competition as age nibbles at our bodies. I've been running more than 25 years, I'm now in my 60s. Aging has a tremendously variable effect on competitive runners. Derek Turnbull of New Zealand, who is a lifetime runner, didn't slow down much for more than 40 years.
Big slowdowns are more typical.
In some cases, slowdown may be due to undiagnosed early disease, such as blocked arteries or cancer, and other symptoms occur much later. Maybe it is significant that Turnbull is a farmer who performs hard physical work every day. Most of the older runners I know have slowed down more the longer they have been racing. It may be that they fail to change their training to allow for effects of aging, or simply don't run as much as they used to. Others may simply run short on psychological oomph.