I would start by studying the big (positive) movements and trends that were pop or maybe even rebellious something around 30 years ago — when your target group was young and restless (twentysomething) — and pick some object or element that you feel to capture the ethos. So like the trends of 1975-1980, depending on how big is the
+ in 50+, you might want to go even further. Music, fashion, TV, movies, maybe even politics are good sources for these.
While rebelliousness may evoke some good, active spirit; 50+ are usually comfort-oriented folk that value easiness over many things — so Dr. Strangelove* references might be double-edged.
Try to be as local as possible. If the global trends from the history book didn't touch your area, don't try to depict them.
Be aware of possible (maybe unintentional) gender biases: being part of a gender might be more of an issue for the 50+ than it generally is for 20+.
Also try to choose something that belongs to their generation and their generation only. For example, copyright issues aside, some sort of a "Jedi Club" -reference would work better if there was no later franchise (movies, games, cartoons, you name it) — while the original trilogy started in 1977, it may feel too 2000ish than 1975-1980ish.
*) Oh well, it's from 1964, but you get the idea