First of all, you don't need to be a movie star or the human pretzel to do it, though many of the poses are pretty challenging. You can usually modify them to a more advanced level if you are able. In yoga, even standing and lying down can be valuable experiences. Sun salute, flying locust, crocodile rest and child's pose become part of your vocabulary. My impression is that mental and emotional control and proper breathing are the most important components.
Yoga is a process, and one learns along the way about form, meditation and relaxation.
Sometimes I have a breakthrough where I become more agile in a pose, or I'm able to discipline my mind more easily. I once learned a lesson about the importance of mental control while practicing when I spent most of a class mulling over a problem, and I couldn't believe the difference by the end. People usually leave class with a contented demeanor, and I recognized that day that, though I had done every pose, I was less relaxed because I had ignored the mind and body connection. The yoga instructors each have different styles and varying levels of knowledge and ability. Thus classes are varied, sometimes more aerobic, other times more slow and relaxing.