system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What happens physiologically that causes men to get man-like features, and lose boyish features as they age?

To be more specific. I have been working with lot of 18-19 year old boys lately, and they are all baby faced. Then, looking back at pictures of myself at that age, I had the same thing happening.

What happens in the mid-late 20's that causes facial features to "harden" up, and lose that soft boyish look?

By 25 you can build the body of a fully mature man easily enough, but facial features tend to stay soft.
gpt
Testosterone. 

Testosterone causes bone to become denser and this alters the shape of the jaw. It also causes you to put on increased muscle mass. 

You also have the natural loss of "baby fat" in the face and neck that both genders go through. These are fat deposits that generally stop being used by the body once you reach adulthood. Instead your body starts putting fat in other places such as the gut and hips.