Although young adults are generally healthy, by their early 20s, a natural physical decline brought about by increasing age, has already begun

Senescence

Explanation:

Senescence is defined as the gradual decline of functional characteristics. It can either be a senescence of the whole organism of just cellular senescence. In this process, cells stop dividing and just enters into permanent growth arrest stage without the cell actually dying. It can be caused by unrepaired DNA damage, increasing age, or other stresses to our cells.