i played intense sports in high school and a bit in college. the sports i played, you will be considered extremely old at the age of 30.
i would say generally speaking in the last 20s, your abilities go downhill no matter how hard you try. at 35, even more, go down significantly. at 45, you simply cant.
players over the age of 25 need to have skills specific to that sports. there are guys who play the sports i played after 35, they cant run as fast as other players 10 years younger than them, but they are still in the game because of specific skills they have acquired over the years.
you are talking about running, so might be slightly different.