I'm a 46 year old green belt who first started TKD (ITF) eight months previous, with no prior martial arts training experience.
Because my dojang is a block from where I work, I attend a lot of my school's noon time All Rank Adult classes. We get a few younger teens in this class, but it's mostly people my age or in their 30's and 50's. One thing I like about this class, is that because almost everyone has to go back to their jobs afterward, it's taught more from a technical standpoint. Patterns, self defense, some low impact drills and bag work. Not very much cardio or intense drill work. I also regularily attend advanced classes for a more intense workout, but the noon class lets me work on fine tuning everything.
My instructor is big on stretching beforehand... probably a good 15-20 min of class is stretching and warmup... regardless of whether it's a beginner class or advanced. Warmups are usually sabhangs. Sabhangs are a set of three movements and are done in four direction succession. We have five sabhangs, each with a specific set of movements.
After the stretching and warm ups, i'm ready for anything... including sparring.
Every now and then, if the instructor is busy, he'll have a young BB open class. And almost invariably, the young gun will just jump right into push ups, or squats, or some fairly intensive(for the old folks) cardio drills... with no stretching. I've never mentioned this to the instructor because it doesn't happen all that often. If it did, I would say something. You have a class full of 30-40-50 year olds of varying rank... there'd better be some pre-stretching involved. JMO.
So, from my limited experience, in both TKD and being athletically involved, I would say the most important part is a good pre stretch and warmup. After that, common sense should be the rule.... such as expecting 40 year olds to do 50 jump spinning kicks on the bag might not be a great idea.