Age is not a period of time but a state of mind. As we do more and more yoga practice, thankfully, we get younger and younger. But, since you asked .
I have students who have truly taken care of their bodies as they have aged, eating healthy, organic food, making sure they were hydrated, and living away from cities so they have less exposure to electromagnetic fields and toxicity. When such students are in class, I certainly can push them harder than I can others of the same age who have not taken care of themselves. Thus, you need to evaluate the intensity of effort on a case-by-case basis. With this said, I can offer some general guidelines.
The basic rule is to make the practice more intense and less showy. This means that, as a student get older, she has to use her mind and breath, not just her muscles, to move her body. The practice becomes less about performing a posture or jumping around to release the frustrations of youth, and more about self-discovery. (And of course, it should never have been about performing and jumping around in the first place!)
Research shows that a body asked to do something gradually is able to improve in strength, flexibility, balance, and endurance at any age. I once read a report about people in their nineties who were asked to begin weight training for the first time in their lives. They started with small weights and, within a couple of months, they increased their lean body mass by more than 25 percent. So it is not that we need to fear physical intensity as our students grow older. We simply need to be more attentive and patient.
In general, older students doing poses that require balance, such as Vrksasana (Tree Pose) and Ardha Chandrasana (Half Moon Pose), should use a wall to prevent falling and injury. Similarly, while older students are doing backbends, be especially careful to see that they do not jam their lumbar spines. Take even more care in forward bending, since the discs of the spine are more vulnerable while bending forward than when bending backward. Make sure your students do forward bends by tipping their pelvises forward by the extension of their hamstrings, rather than bending forward by rounding their spine. When a student starts to feel pain during a backbend or forward bend, ask them to back off and do a lesser pose with their mind and breath, creating a lengthening of the spine and an opening, rather than pushing their body into the pose.
Usually a student who has practiced from youth to old age must be treated differently than an older student who started later in life. For example, if a student is not used to doing Padmasana (Lotus Pose) and you introduce this pose later in life, you have to be especially careful that their hips are open enough, or else they will take the strain in their knees.
One more important tip: If the student is not hydrated enough, you will find that her skin is more wrinkled, her tendons will tear more easily, and her discs are more vulnerable to rupture. So encourage all your students, but especially those getting on in age, to drink lots of fluids.
Recognized as one of the world's top yoga teachers, Aadil Palkhivala began studying yoga at the age of seven with B.K.S. Iyengar and was introduced to Sri Aurobindo's yoga three years later. He received the Advanced Yoga Teacher's Certificate at the age of 22 and is the founder-director of internationally-renowned Yoga Centers in Bellevue, Washington. Aadil is the director of the College of Purna Yoga, a 1,700 hour Washington-state licensed and certified teacher training program. He is also a federally certified naturopath, a certified <a href="/health/ayurveda">Ayurvedic health science practitioner, a clinical hypnotherapist, a certified shiatsu and Swedish bodywork therapist, a lawyer, and an internationally sponsored public speaker on the mind-body-energy connection.