You are correct -- at 45 I will be the second youngest paying passenger after you! Haha! Be prepared for a lot of old Jewish ladies kvelling that a young person is interested in jazz and on the cruise. Of course lots of the musicians, especially the side people, will be in their 20s and 30s.
As far as advice goes:
Two of the three main venues, the Sky Lounge and the Rendezvous, are pretty small (and not all of the seats in the Sky Lounge are that great). If you have a must see artist it's worth arriving a little early. Generally you can always get a perfectly good theater seat even once the show is in progress. The big band shows in the theater (which are fabulous -- all the headlining artists on the ship rotate sitting in with the big band) always draw a good house but it's not like you have to line up an hour in advance to get a seat or anything like that.
Sometimes the place will be jammed and sometimes, if there's a big show elsewhere, it won't be (or the crowd will filter out as something else starts up so you can upgrade your seat during the show). Last year on one of the at seas days they had some great artists playing a Spinnaker show at 1, and then another great group playing at 3, and really not much else going on. When the 1pm show ended no one left, and there was a huge group of people trying to get into the show as well, so they moved it down to the theater which was a wise move. I think they're going to do more counter programming this year so that doesn't happen. On the first night you can try checking out a show at each of those venues so you can get the generally vibe and plan accordingly. The Welcome Show is great because a lot of the artists do a song or two so you can get a nice taste of who you want to prioritize.
One thing about showing up early is that you can chat with the people next to you so you'll have a familiar face the next time you see them around the ship. If you go to the main dining room without an assigned table you'll get randomly placed with seven or so other people. Some may find this a great way to make new friends, and some might prefer to just hit the buffet instead.
I'll try to think of some more advice.