Modern athletes in heavy sports (judo, weightlifting, boxing, …), are classified according to their weight. In antiquity, weight categorizing was unknown; only age categories existed. At the beginning, there were two age categories: "boys", with age limits roughly equivalent to the teens and "men".

The minimum age for participation in the games is estimated at 12 years. At that ages, boys could have had years of athletic education. The youngest known participant in the Olympic games, was Damaskos of Messene. In 368 BC, he was proclaimed victor in the stadion for boys at the age of 12. The number of events for boys at Olympia never reached more than five, a number lower than that for men. The other games of the periodos, had more events for boys. In the third century BC, most games introduced the category of the "ageneioi" (literary "the beardless") for athletes in their late teens. The Olympic and is-Olympic games never adopted this category; they retained the two basic categories of boys and men, the latter category referring to those of 18 year and older.

From the 3rd century BC onwards, the boys category was further differentiated. Most widespread were the "Pythian boys" and the "Isthmian boys", referring to the way in which the boys were classified in the Pythian and Isthmian games respectively. The exact age distribution is far from certain. One proposal is that of Klee: "Pythian boys" between 12 and 14 years;"Isthmian boys" between the age of 14 and 17 years; "ageneioi" between 17 and 20 years.

Age	12	13	14		15	16	17		18	19	20	21	22	23	24	25	+25
Olympia
Other

	Boys
	Pythian boys
	Isthmian boys
	Ageneioi
	Men

In Roman times the boys were sometimes divided into three classes: younger, middle and older. In some games, young participants were allowed to participate in higher categories: Artemidoros from Tralles, who lost the pankration for boys at Olympia, later beat the boys, the ageneioi and the men in the pankration in the games of Smyrna, all on one and the same day.

Distinguishing the age categories was often problematic, as there were no birth certificates or identity cards; the age had to be determined on other criteria, such as size and beard development. In 468 BC, Pherias from Aigina was thought to be too young for the wrestling competition, and was not allowed to compete. At the next olympiad he was admitted to the wrestling for boys, and won it.

Boys in the Pythian games used a smaller discus in the pentathlon than adults. Prizes could also differ according to age categories. In the Great Panathenaic games victors were ranked according to event and age category. The prizes for the ageneioi were somewhat higher than those for the boys, but markedly less than those for the adults. Finally, geographical mobility was also different between age groups. Boys mainly originated from the nearby cities; the events for the adults were far more international.