A home grown player is defined as one who, irrespective of his nationality or age, has been registered with any club affiliated to the Football Association or the Welsh Football Association for a period, continuous or not, of three entire seasons or 36 months prior to his 21st birthday (or the end of the season during which he turns 21)
For the purposes of this rule, the season is deemed to run from the date the first transfer window closes until the final match of the campaign.

In keeping with UEFA guidelines, an Under 21 player is defined as one who is under the age of 21 on 1st January in the year in which the season commences. For the 2010/11 campaign Under 21 players will have been born on or after 1st January 1989.
EXCEPTIONAL CIRCUMSTANCES
The 25-man squads must be submitted to the Premier League by 5pm on the day following the close of each transfer window. For the 2010/11 season the first window will close at 6pm on 31st August and final squads must be registered by 5pm on 1st September. Before then Clubs will not be required to name their squads and may select their teams from any registered players.
While the transfer windows are open, clubs may change their squads as many times as they want. However they will only be able to make alterations outside the windows if there are exceptional circumstances and the changes are approved by the Premier League board.
For example, if a club has two out of three goalkeepers injured they may be permitted to bring in a replacement under the guidelines drawn up by a specially formed working party.
Players in the 25-man squads who go out on loan to Football League clubs may not be replaced. However they may reclaim their place on their return.
Clubs do not have to name a full squad of 25 if they do not have that many contracted players and in this instance they may add free agents outside the transfer window. However if they have 25 eligible contracted players, they must be named.
Similarly, clubs do not have to name eight home grown players if they do not have that many but that would mean operating with a reduced squad.
The system will be simple enough to administer. Clubs will register their players via an online form which automatically flags up if they have named too many players who do not qualify as Home-Grown.

Premiereleague.com