I was about to ask exactly the same question. DS is 17 weeks and craning his neck backwards in the pram to see out the "window" in the hood behind his head (Pliko Pramette). I think he's ready to have a proper look at the world.
We converted the pram to a stroller at the weekend, but DS wasn't too impressed with the wind in his face (forgot to bring out the rain cover, doh). I also wonder if he missed looking at his mamma. It does lay flat in stroller mode if you want, but I decided that I'm not ready to not see my bubs when we're out walking. He looked so tiny in the stroller that I'm happy to leave it for a little longer.