Quote:
"You can take your theory test once your provisional licence becomes valid.
Normally, for car drivers, the earliest date your provisional licence can become valid is your 17th birthday, however you can apply for the licence up to three months before your 17th birthday.
So, if you apply for your provisional licence before you are 17 you will still have to wait until your birthday before you can take your theory test."
Source:
http://www.dsa.gov.uk/General.asp?id=SX1157-A7 7F5778&cat=2
Chris