I pulled some excerpts from about.com's site written by Jamie Fitzpatrick which you can see in full through the title-link.
Most Pertinent to College Hockey:
Players who turn 18 by September 15 and are not older than 20 by December 31 are eligible for selection. In addition, non-North American players over the age of 20 are eligible.
A North American player who is not drafted by the age of 20 is an unrestricted free agent. All non-North Americans must be drafted before being signed, regardless of age.
Re-entering the Draft
A player not signed by his NHL team within two years of being drafted can re-enter the draft, as long as he is 20 years old or younger at the time of the subsequent draft. Players over 20 become unrestricted free agents.
NCAA players are an exception: NHL teams retain the rights to a college player until 30 days after the player has left college.
A team that does not sign a first-round draft pick is awarded a compensatory pick in a future draft upon losing the rights to that player.
A player who has been drafted a second time cannot re-enter.
And this recent change:
NCAA Players - As of 2004, 18-year-old players from NCAA Division I schools can be drafted and retain their college eligibility as long as they don't play for a pro team or hire an agent. In previous years, an 18-year-old who opted into the draft lost his NCAA eligibility.
So, you can see that Phil did not turn 20 until March 19th (Happy Belated Birthday Phil!) so he WAS still draft eligible and it paid off. There were other Wildcat moves around the Washington Capitals organization as Bryan Muir(92-95), who was only one of two Caps that possess a Stanley Cup ring(see his name on the Cup), was let go after having missed much of last season after suffering a broken foot, so we'll wait to see where he lands - hopefully he gets a chance to build on his 05-06 breakout year. And Rod Langway(75-77) was in attendance at the Capitals Draft party signing autographs.