Read Maty Ezraty's response:
Students come to yoga with different and unique situations, personalities, and physical limitations and attributes. Yoga practice is for everyone, but not every pose is appropriate for every student. How you do is more important than what.
In other words, you should teach yoga to people and not teach people yoga. Older students and students with medical concerns should be treated with extra care. The traditional Ashtanga practice may need to be tailored to meet their individual needs.
Nevertheless, there are 40- and 50-year-old students who are perfectly capable of practicing Matsyasana (Fish Pose) safely. Longtime students of this age may be capable of many poses that latecomers to yoga should avoid.
Conversely, there are 20-year-olds with neck injuries who should not attempt this pose. The student's age must be taken into account, but more it is important for you, as the teacher, to observe the individual and his or her physical condition.
When done poorly, Fish Pose can put too much stress on the neck, so it is often not recommended for beginners. Fish Pose and the final posture in the first series, Setu Bandasana (top of the head on the floor, hips off the ground, feet turned out, legs straight, and arms crossed over the chest) require an open chest so that the neck does not bear the brunt of the pose. Poses such as Trikonasana (Triangle Pose) train the neck to look up at the hand and help strengthen the neck muscles. Simple backbends open the chest and support the pose so that it does not become a neck crunch.
As for students with heart problems, I would be very cautious about teaching them the full primary Ashtanga series. I would be more inclined to teach them a slower version, and to begin with I would avoid overworking or overheating them. I would modify the program considerably, omitting many if not all of the jumps and doing a shorter practice that includes lots of restoratives. I would also encourage them to consult their medical doctors, and I would remind them that I am not a medical expert.
Once, in India, I watched K. Pattabhi Jois work with a woman who had diabetes. He omitted all the jumps and was very careful with her. He allowed her more time to get in and out of poses and treated her with lots of love and care.
Maty Ezraty has been teaching and practicing yoga since 1985, and she founded the Yoga Works schools in Santa Monica, California. Since the sale of the school in 2003, she has lived in Hawaii with her husband, Chuck Miller. Both senior Ashtanga teachers, they lead workshops, teacher trainings, and retreats worldwide. For more information, visit http://www.chuckandmaty.com.