Answer:The prepositional phrase in the sentence "he started college at age sixteen" is "at age sixteen." It begins with the preposition "at" and ends with the noun phrase "age sixteen." This prepositional phrase provides information about when he began college, specifying his age at the time. The preposition "at" indicates a specific point in time, in this case, the age of sixteen. The entire sentence communicates that he began his college education at a very young age.