It's your argument that he's too young, despite there being no evidence this has held him back and plenty of contrary coaching successes, like Joe Brady.
And Knute Rockne, who was a ND assistant for 3 years before taking over as HC.
Ageism goes both ways. Professionals care less about age, and even credentials. It's about results. PPG rankings are useful, but not the only argument. The fact is, Rees, in his first season as OC, led an offense that won all regular season games, played a credible run scheme, and fit perfectly with the starting QB strengths.
As a former military officer and combat veteran, I can tell you 28 years old is plenty old enough to lead young men and old. You're either capable or not. A good leader or not.