The debate over the merits of the one-and-done player has been brewing since the NBA closed the draft to high-school prospects in 2006, when it raised the eligibility age for prospects.
Since those rules (see page 295) were implemented, the policy has stated that a player to be eligible for the draft:
(i) The player (A) is or will be at least nineteen (19) years of age during the calendar year in which the Draft is held, and (B) with respect to a player who is not an international player, at least one (1) NBA Season has elapsed since the player’s graduation from high school (or, if the player did not graduate from high school, since the graduation of the class with which the player would have graduated had be graduated from high school.