I see clarification for the question now…. for middle aged..
Age is not the concern as much as their previous experience. No advanced pose should be offered unless the teacher really knows the ability of the students. Otherwise, the class is geared to allowing the students to feel they were successful and enjoyed the experience.
The term “Classical Ashtanga” probably means different things for most individuals who are unfamiliar with the true traditional-classic meaning of Ashtanga. If one were to study and practice under the “classical” Ashtanga method, the class would be termed “Mysore” or a variation – led or guided class. Either of the latter yoga type scenarios would probably be difficult to garner great success in a fitness/health club setting. However, other Ashtanga styled yoga variational classes that have a vinyasa flow aspect incorporated into their pose sequencing can be very successful in fitness clubs if the yoga strategy portion of their business is well constructed and implemented.
I taught in health clubs for over a decade, and had classes of the Primary Series of Astanga Vinyasa. Most of the students were 40 – 55, and dedicated enough to learn to practice on their own, incorporating a home practice in addition to classes. There are a couple of issues…in a continuing class with experienced students, it may be difficult for a newcomer to intergrate easily, unless you have an introductory class just for beginners.
These groups of students were very dedicated, and learned how to modify many postures to their own abilities and needs, as they had been practicing with me for some time and had a good foundation. It is also important to consider the mix of clientele..many postures are not appropriate in the event of certain medical conditions and physical limitations, and can cause harm. It is important to understand the effects of each posture on the body.(Example: people with glaucoma, and uncontrolled high blood pressure should never go upside down – head below the heart). So to teach this system in a health club, you need a strong knowledge of this aspect of the postures, how to modify for various conditions, and what variations are appropriate.
The main issue I found within that particular system of hatha yoga is that unless the postural alignment is impeccable through the down dog / chaturanga / up down portion, students can and do develop issues with the shoulders, particularly the rotator cuff. The goal of all yoga is to heal and help, not harm. Various stages of life can utilize an ever changing practice to support us on our journey.
The topic has been discussed elaborately in this thread. Great job by all the contributors!