first timer or age grouper?

It's really up to you.  Chances are you'll have a better chance of a higher ranking if you register as a first timer.  But that may not be fare.  If it were me, and I had raced a couple of sprints, then I would register as an AG'er. 

Usually the race web site will state the requirements for registering novice.  The one race I did stated that you couldn't register novice if you've done 3 tri's.  But some races don't specify.  You can alwasy ask by emailing the race director.

It's up to you and what you're more comfortable with.  I would think it's ok to register novice since you haven't done this distance before.  But you may not see yourself a beginner because you've done a few sprints.  It's up to you- if you think it's fair, then register novice.  If you truly don't feel like a beginner, register AG.  I think it comes down to the honor system because I don't think anyone really checks.  I don't think it would be wrong to register novice for your first olympic- as long as you don't keep doing that.