Question

How does the structure of the dermis change when skin ages?

Answer 1

In all tissues where cells need replacement have stem cells. These stem cells generate new cells.

Explanation:

Each tissue is having its replacement cycle. Epidermis and dermis are exposed to stress and worn out. When old cells die new cells are generated through stem cells.
Not only dermis almost all organs have stem cells to replace old cells. Exception is brain cells. Nerve cells have lost the power of division are not replaced easily.
The cells of epithelia are replaced at a fastest rate.