When does the most significant drop in physical activity typically occur in people's lives?

Later high school years and freshman year of college

Explanation:

The transition from high school to college is an important developmental milestone that holds the potential for personal growth and behavioral change.  A cohort of 2,025 students was recruited during the summer before they matriculated into college and completed Internet-based surveys about their participation in a variety of behavioral risks during the last three months of high school and throughout the first year of college. Alcohol use, marijuana use, and sex with multiple partners increased during the transition from high school to college, whereas driving after drinking, aggression, and property crimes decreased. Those from rural high schools and those who elected to live in private dormitories in college were at highest risk for heavy drinking and driving after drinking.