I am 61. I do not know if I have osteoporosis or osteopenia. After being chemically poisoned, I assumed I did so refused to have a test that would make me feel I had to be 'careful'. I had enough problems with lung disease!
I have been doing yoga (without the benefit of a teacher) for 4 years. (I started at age 57 with severe lung problems.) Most seniors have health issues. This is what brings most of us to yoga.
I have used many dvds over the past 4 years, including Kripalu, Ganga White, Yin Yoga, etc. Each gave me a 'piece of the puzzle'. The person who helped me to best understand the need to HOLD poses long enough to move slowly enough to allow the pose to unfold was Erich Schiffmann. His book (MOVING INTO STILLNESS) opened up many windows of understanding. I now see forward bend as an asana in its own right...not as part of a sun salutation or as part of a total program. I understand the need to slowly allow my body to open to this asana. Before reading Schiffmann and watching his DVDs, I was only doing forward bends as part of a total program (sun salutations or Kripalu Gentle, etc). Now I rarely do a full forward fold except as a practice just for this as my emphasis is on using this asana to 'lengthen my spine' as I fold. I purchased a mirror and watch as I fold to insure my spine is straight. As soon as it rounds, I stop (as Schiffmann demonstrates) and allow my hips to open more. I now understand that it is okay if it takes my body years to achieve a full forward bend (as a separate asana)! Only after I able to do a forward fold with a straight back will I use it as part of a vinyasa. Until then, I do simpler vinyasas (such as plank to chaturanga to cobra or up dog to down dog or child's pose).
When teaching yoga to seniors, IMHO, the goal is help us understand that vinyasas (which are so emphasized) are not as important as restoring atrophied LM and maintaining flexibility. This is achieved in stages over years...not months! It will take us much longer to achieve the same results as a person even in his/her 50's! However, we often push ourselves too hard as we try to keep up with those who are 10 years younger! Menopause will make a huge difference in women. We can progress much faster BEFORE menopause than AFTER menopause. Therefore, it would be wise to know if the women in your class are pre-menopausal, menopausal, or post-menopausal.
Although I have used many yoga DVDs (60+) and have read many books, the ones I have learned the most from are: Erich Schiffmann's MOVING INTO STILLNESS (and watching his DVDs in which he explains his approach) and Kali Ray's FREEING THE SPINE. These still continue to benefit me as they allow me to 'go back' to the basics...something which is frequently needed as muscles are restored to full activity and the hips are re-opened.
Although Yin Yoga is VERY beneficial for senior, go slowly. Atrophied muscles will need to be re-activated and joints, unusued for many years, will need to be VERY GENTLY re-opened. As the joints are opened, waste matter which has accumulated in them FOR YEARS will be released. This will cause exhaustion about 24 hours after the practice as these toxins circulate in the bloodstream! Therefore, I would suggest that you not have your seniors do it more than once a week until they are past this stage. They should drink LOTS of water for the first 24 hours after the practice. Also, they should NOT do a yang yoga practice the next day IF they are feeling tired! After doing Yin Yoga several times, the release of most toxins will have been accomplished and this will no longer be a problem.
After 4 years of learning and experimenting, I am able to do 45-60 minutes of an advanced beginner's vinyasa. However, I benefit the most from 45-60 minutes of slow rhythmic movements which gently lengthen (and relax) the muscles and static holds that gently 'open' the joints. Even though I do yoga DAILY, this process (of restoring joint flexibility and muscle/spinal lengthening) is a daily RENEWAL process! While progress has been steady, it has also been very slow! I have had to frequently remind myself that "The goal is NOT the mastery of an asana or vinyasa but the daily restoration of joint flexibility, spinal length, and muscular strength. The destination is the journey."
While there are seniors for whom the above information may not apply, it will apply for most...especially those who have not been involved in a formal exercise program beyond walking.