Because people with good jaw projection when they were young end up being a old man at the age of 40 theory

[Arthur schopenhauer]

One of the main factors is the subhuman fat accumulated in the mentocervical angle but this explains why people with good facial development accumulate so much fat, and the definitive reason is due to the lack of bone that comes after 35, and the reduction of bone density, which is why the jaw narrows and the maxilla loses projection.