Here are the rules on the prospects that are and are not eligible for selection in the NHL Entry Draft each season.
To be eligible for selection a player must be at least 18 by Sept 15th of the current draft year. The vast majority of players selected a
Here are the specifics from the NHL.com on what makes a player eligible and ineligible.
Entry Draft Eligibility
All players age 19 or older [(i) any player who will be age 18 on or before September 15 in the year in which such Entry Draft is held, or (ii) reaches his 19th birthday between September 16 and December 31, both dates included, next following Entry Draft, can attain eligibility by delivering to the League a written notice (Opt in Form) prior to the later of May 1, or seven days following the date such player finishes competing on his team in the year in which such draft is to be held.] are eligible for claim in the Entry Draft, except:
(i) A Player on the Reserve List of a Club, other than as a try-out;
(ii) A player who has been claimed in two prior Entry Drafts;
(iii) A player who previously played in the League and became a free agent pursuant to the Collective Bargaining Agreement;
(iv) A player age 21 or older who had played hockey for at least one season in North America when he was age 18, 19 or 20
The 2013 NHL Draft is scheduled to be held from June 28th to June 29th inclusive in Newark, New Jersey.