Had a series of injuries from about 25-35, then changed things up due to concerns about aging. Lighter weights, actually following proper training plans/shoes for running, etc. No injuries since and I'm about to hit 50. Aside from raw strength, don't think I feel any different now than at my athletic peak in my early 20s.

Having lived in San Diego most of my life, I've routinely met people in their 60s doing Ironmans. When you're around that enough, it's clear the difference is training smart than simply training hard.