In a pride of lions, one (occasionally as many as 3-4) male is the undisputed leader. This male is usually between the ages of 3 and 15, and it is he who mates with all the females, drives out the younger males, and defends the pride from outside threats.
Young lions, on the other hand, are driven out of the pride by the old male/s when they reach maturity. From this point on they generally roam, looking for an old male to challenge and replace as head of their own pride.
The analogy extends to human behavior, particularly in areas where men compete, such as sports and social situations. For instance, an 18-year old college freshman continually trying to outchug a 24-year old graduate student who has been drinking for several years may be referred to as having "Young Lion Syndrome."
Headline: Vitali Klitschko knocks out Samuel Peter! Today the old lion returned to defend his crown. 37-year old Vitali Klitschko hammered 28-year old Samuel Peter, forcing him to quit at the end of the eighth round.
Peter declared during a press conference prior to the fight that Klitschko was "too old" to defeat him - clearly a case of "Young Lion Syndrome".