Fitness

This is a terrible study design. A person's physical fitness doesn't remain the same over time. The study looked at health at age 18 and then locked in the groups for the rest of the study. A doctor is going to say, "Exercise now," not "Go back in time to age 17 and exercise so you have good fitness at age 18." What matters is how exercise impacts a person over time, not how exercise from 30, 40, or 50 years ago impacts you today.