Beginning in 1971, underclassmen were allowed to enter the NBA Draft provided they could give evidence of “hardship” to the NBA office. In 1976 the hardship requirement was eliminated in favor of the current Early Entry procedure, whereby any athlete with remaining college eligibility can enter the NBA Draft on the condition that he notifies the league office at least 45 days before the draft. International players could declare eligibility in the calendar year of their 18th birthday, or later.
Starting with the 2006 NBA Draft, the eligibility rules have changed:
1. - All players, regardless of nationality, must be at least 19 years old during the calendar year of the draft.
2. - A player who completed basketball eligibility at a U.S. high school, regardless of nationality, must also be at least one year removed from the graduation of his high school class.
This age limit for draftees is part of the new collective bargaining agreement between the league and its players union.
The NBA has established two draft declaration dates. All players who wish to be drafted, and are not automatically eligible, must declare their eligibility no later than 60 days before the draft. After this date, prospective draftees may attend NBA pre-draft camps and individual team workouts to show off their skills and obtain feedback regarding their draft positions. A player may withdraw his name from consideration from the draft at any time before the final declaration date, which is 10 days before the draft. A player who declares for the draft will lose his college eligibility, even if he is not drafted, if any of the following is true:
The player signs with an agent.
The player has declared for and withdrawn from a previous draft. Although the NBA collective bargaining agreement allows a player to withdraw twice, the NCAA only allows one withdrawal.
When a player is selected in the first round of the draft, the team that selected him is required to sign him to at least a one-year contract. Teams own the rights to players selected in the second round for three years, but the teams are not required to sign them.
Source: Wikipedia, NBA.com