Your NI card has been posted to you so that you'll have it ready when you start full-time employment after you've officially left school (i.e. after 26 June 2009). Until then you can only work for a limited number of hours per day (and limited number of hours per week in total) and only in jobs approved by your local education authority (and with a permit provided by that authority):
http://www.direct.gov.uk/en/Parents/ParentsRig hts/DG_4002945
The 26 June date applies to everyone in your school year group, irrespective of whether they've been 16 for many months already (e.g. since last September) or won't be 16 until well after that date (e.g. at the end of August). You all become eligible for full-time work at exactly the same time.
Chris