McKinsey's MO is to have consultants work for a few years and then become "Alumni" who work for real companies and/or become secretaries of transportation where they can hire McKinsey to screw more things up. This is the reason the consultants are almost entirely in their mid-20s.

"are almost entirely in their mid-20s"

Probably a lot easier to keep their salaries relatively low as well?

The entry-level McKinsey consultants that I know began at $250,000/year; they didn't have other places they could get those size offers. Maybe $180-200 at most, if they could wrangle a high-value position at a startup. I'm sure the salaries have only increased since then.