The guy at Pacific Cross I bought my insurance from confirms that they will sell insurance to people over 70. "Yes it is possible just the age bracket will be different ... Same as you we typically require the health check up but can do it when he arrives in Thailand." I can provide a contact if that is helpful to anyone.
I'm 67 and I have just bought the required health insurance to return to Thailand. I don't think there is an upper age limit, but if you are over 65 you will have to pass a fairly rigorous medical examination, as I did. I would think the older you are the higher the premium you will have to pay. Insurance companies are not charities, but if they think you are a reasonable risk they will sell to you.