Crystal has an average age of 60 + and while there will be some in your age bracket, there won't be as many as you find on the mass market lines. Crystal doesn't attract many families,so that goes a long way in keeping the average higher. While the average might be a little lower in summer, it won't vary by much.
Crystal is also one of the most dressy lines and that seems to keep a lot of younger passengers away. It seems that the younger passenger wants to cruise in only casual clothes and don't want to dress up on their cruises. On a typical cruise men wear tuxes (at least 90%) and the rest wear dark suits for formal nights. On informal nights it's dress slacks, dress shirt and jacket (ties aren't required for informal nights but many men chose to wear the tie) and casual nights are collared shirts and nice slacks.
If you're looking for a more relaxed cruise, then Crystal is for you. They don't do what all the other cruise lines do like horse races, pool games and activities like that. In the evening they have a production show or guest performers like classical musicians, vocalists or comedians. There are three other places for dancing or listening music. You won't find rock music or anything like that.