Why are adolescent athletes more prone to concussions?

Multiple reasons. Aggressiveness of activity types of unprotected head strikes lack of protective equipment- helmets, etc fullness of the brain within the cranial vault many other reasons.
Impulsivity. Adolescent athletes tend to be more impulsive than children and adults. They take more chances due to their impulsivity and therefore put themselves in the situation to get concussions more than people of other ages.