Ehhh..... It's more complicated than that, at least in regards to bone and joint health.
During your thirties is when hormones tend to slow down and people start to lose some of their muscle and bone density. Even if you remain active, you will still degrade in musculoskeletal health, just at a slower pace.
Injuries do heal slower, and you start to acquire overuse injuries. Which is why so many people in their 40s are starting to have to have total and partial knee replacements.
I work in orthopedics and rehabilitation, and see a ton of marathon runners for chronic knee, ankle, and foot injuries. I highly recommend people migrate to more low impact cardio activities for exercise before their 40s so you can maintain your mobility later in life.