Try telling that to Lagat. The guy has set PRs into his late 30s in the 5k, and routinely thrashes athletes who are almost half his age.
Lagat runs 50-60 mpw, and touches on 70 in base. He runs once a day 6 days a week. His only cross training is core work once a week.
Rupp runs 110-120 mpw with an extra 20 mpw underwater and lifts twice a week. Rupp ran 1337 the summer after HS on 50-60 mpw! Why in the world does he increase his volume? Seriously!!!! He should have kept his training the same as HS senior year and just continued to do the workouts faster each year as he improved, like Lagat!
Comparing their workload, of course Lagat's been around a long time and of course Rupp's time is limited.