Yoga classes for the elderly are adequate to their abilities, working within the limits of what is allowed by the body and always aiming to improve the quality of life of the elderly in every way. It is always good to keep in mind this practice, which is never late to start.
Generally, a senior yoga class lasts just like any other. Of course, this always depends on who imparts it. But between an hour and 90 minutes is a time that is accustomed.
Some postures, obviously, are much softer and quieter than those of a yoga class for younger people. It is that not all people at that height have their bones and muscles in such conditions.
However, what you are looking for in a senior yoga class is to stimulate muscle flexibility, irrigation and posture. The body will oxygenate in a great way and, progressively, it will be possible to work with exercises of greater complexity.
It is important to first consult with a doctor if you have bone or muscle problems. He will surely know how to explain the postures that can and can not be done. Beyond this, it is always a good opportunity to start practicing yoga. Be as old as you are.