What is the most obvious indicator that aging begins in young adulthood?

There is a lot of evidence that agings begins in young adulthood. I would say that a lot of people begin the lose their hair during young adult hood, or there is some grey added to their hair. It normally is a pretty slight change. For example, I noticed a had a couple of grey hairs in my beard in my early twenties, but they were hardly noticible.

True statement gravity force is more on bigger mass, less on smaller mass.all mass fall on planet with same acceleration and speeds