Why are there so few old ICs?

I work at a huge tech company.

In my org of 100, the rough breakdown is like this:

70: IC under age of 40 25: Managers, mostly over 40 5: IC over 40 (all are principal)

I know people say that as you age, you move to management or principal. But the math doesn't check out.

In my scenario, if we broke it down like this - 30 openings for traditionally "over 40" roles - 70 people that will eventually be "over 40"

70 people - 30 openings = 40 people without jobs

Where do they go?