The parallel with running is flawed. Cycling is easy on the joints and doesn't require the same elasticity - "bounce" - that, when lost, turns a runner into a plodder. In other words, cyclists need the engine, but don't put as much stress on the transmission.

Last year while racing I met a nice gentleman who was bemoaning the fact that the previous day he had performed poorly in a 40k time trial, having finished in 64 minutes. He was 75 years old.