The $1B company that prefers 40 year old coders who only work 45 hours a week

Because in silicon valley and the startup scene in general you will see mostly 20 somethings as software engineers. Companies like that they can pay them less and get long hours out of them . (No children, work is #1 priority, etc).

Possibly. The programmers I work with in that age range seem to make it a point to write hard to read and understand code. This in a way keeps their jobs more secure and in general has made it tough to onboard new programmers.