Hi, Youngcouple, and welcome to LCT! My husband and I have been on the PG twice and I know that you will love it. We were in our late 50's, early 60's when we went and I'd say we were a bit beyond the median age range on our sailings. As Karen and jhp mentioned, the PG draws lots of honeymooners so there will be plenty of other young couples on board.
We're avid snorkelers so we booked every snorkeling excursion that was available. Our first trip was in August/September 2004. The second trip was in June 2005 which was just after they began allowing early booking online for specialty restaurants and we took advantage of doing this. We booked our excursions online for both trips. You are not tied into what you book until 24 hours or so before the excursion so there is time to change your mind after you board the ship.