I'm focused more on experience than age, and Knute & Leahy were from such a different era that it's hard to extrapolate to today's coaches.
Up and coming coaches that had little to no head coaching experience (such as Faust, Davie, and Weis) were ill prepared and they got so chewed up and spit out that they never had any notable success thereafter.
Meanwhile, some of the established coaches that they hired (such as Devine and Holtz) went on to fame and success - though Devine probably underappreciated in the shadow of his predecessor.
Then you have guys in the middle like Ara, Ty, and Kelly who had some head coaching success at weaker programs from major conferences. With those guys, it takes good judgment to pick a winner like Ara.