Young T. rexes had a powerful bite, capable of exerting one-sixth the force of an adult: Juvenile tyrannosaurs tested their chops as they grew to become bone crushers like their parents — 

Scientists have experimentally measured the bite force of adult T. rexes but not of younger tyrannosaurs. Fossils with juvenile bite marks have now allowed experts to experimentally test how hard juveniles could chomp. Though their bite force is one-sixth that of an adult, it is still stronger than that of living hyenas. The measurement is higher than previous estimates, suggesting a different ecological niche for these youngsters.

Source: Young T. rexes had a powerful bite, capable of exerting one-sixth the force of an adult: Juvenile tyrannosaurs tested their chops as they grew to become bone crushers like their parents — ScienceDaily