Read Aadil Palkhivala's response:
I recommend taking a teacher training course in an alignment-based method such as Purna Yoga or Iyengar Yoga. Since injuries can happen more easily to seniors than to younger students, correct alignment becomes critical when teaching them.
A good teacher training program will not only give you information on how to do poses but also on how to teach them to a variety of different people. Please search for a school or college that has a program of more than 500 hours. I say this because ever since yoga's popularity began to surge, standards have been plummeting, and you can now get a teaching certificate in a weekend!