Just gonna drop my 2p's worth in here, I hope nobody minds as I was in exaclty the same position when I passed my test some years back.
I was 27 when I passed my test (over 5 years ago now

) on a GS500 ... after passing a week or so later for my first bike I got myself a Ducati Monster S4 which had a very meaty 916cc motor straight out of the Ducati 916 Superbike ... this was a very quick bike with some tremendous torque the kind that you can only get from a V-Twin.
I've just got myself a Ducati 1098s as I'm getting back into biking after a couple of years out but thats another story as owning one Ducati has got me properly hooked on them now, only downside is the cost of them

.
What you buy depends on you as a person, I know if I'd have leart to ride a motorbike when I was younger (early 20's) I would have been a hoon, however I learnt when I was 27, I was old enough to be a bit more sensible about things and knew I could trust myself on a more powerful bike.
No matter what advice you are given, chances are you'll go and buy what you want anyway its pretty much always usually the way which is fine as long as you do it sensibly and think it through, end of the day you have to like what you are riding otherwise the appeal goes away.
My advice is if you can trust yourself and know you can be sensible and enjoy a bike like a CBR600F or whatever you choose within sensible reason then go for it, the throttle will only open as far as you want it to ... same as in a car, it's really all about you.
Stu