There's one theory that says, essentially, that we all have a certain number of running miles and when you reach the total that's it.
Haven't made up my mind on this. As it seems way oversimplified. Yet there could be some truth.
My own running was nipped in the bud at 19 as i contracted chondramalacia. And from then on decided i would just run for fun and to be healthy. And to learn how to go the distance. And it's never too late to learn a good thing.
My recent (5 years) barefoot running experiments have "evolved" my pleasure&health-driven approach to include amateur competition and conventional training methods. When you experiment with anything, it is a good idea to have some reference points and competition and racing defenitely offer some.
But the most interesting area of this experimentation has to do with re-generation: of tissue and will alike.
The soles of our feet regenerate with recovery. The souls of our minds need to do the same. Old shoes wear down and that's it. They are thrown away and you have to buy new ones. But we can recover.
My feeling is performance-driven obligation leads to injury. Pleasure&Health-driven competition leads to well-being and healing.
So far so good and improvement continues against the aging curve. But the Olympics and Masters championships podiums are still out of sight. The podiums, but not getting to the starting line...