NBA
College players who have completed their four years of eligibility are automatically eligible to be drafted. Underclassmen must declare eligible for the draft, give up their remaining years of college eligibility in the process. International players who are at least 22 years of age are automatically eligible, and those younger than 22 must declare. Prior to 2006, high school players were eligible to be drafted. Now, a draftee must be one year removed from high school, and 19 years of age at the end of the calendar year of the draft.
MLB
Draftees must be a resident of the United States, Canada, or any other U.S. territory such as Puerto Rico. Players from other countries are eligible to be signed by any team, and are not subjected to the draft. To remain draft-eligible, a player must have never signed a major or minor league contract. High school players are eligible after graduation. Players at four-year colleges are eligible after their junior year, or if they are within 45 days of their 21st birthday. Players at junior or community colleges can be drafted at any time.
NHL
Players must be 18 years old by September 15, and no older than 20 by December 31 in order to be eligible for selection for that year’s NHL Entry Draft. Non-North American players over the age of 20 are eligible as well. Players not within those restrictions (i.e. North Americans older than 20 years of age) can be signed as unrestricted free agents.
NFL
Players are eligible if they are three years removed from high school. While a player doesn’t have to attend college to be drafted, nearly all draftees played college football. A year as a redshirt counts towards eligibility despite the fact that redshirted player is inactive for that season. Thus, a player can be drafted after his redshirt sophomore season.