Mike Tomlin stepped into a unique situation when he was hired at 34 years old as the head coach of the Pittsburgh Steelers in 2007. He was roughly the same age as some of the players and he had a roster that was battle tested and ready to compete for another Super Bowl championship after just having won one two seasons prior. Cowher left not because he was fired, but because his wife had fallen ill and he wanted to spend more time with his family.
Normally coaching transitions come when a team is bottoming out, and the new coach is tasked with rebuilding the team from the ground up.