From my understanding its quite common that thai people make life/health insurance for the elderly in the family. 

Mother in law is 70.

Would it be worth it or the premium is to high at that age? 

There is some form of life insurance that is available through the amphur or tessaban but I don't know the specifics, my wife tells me that her mother has such a policy. But main stream insurance companies in Thailand will be reluctant to offer policies to 70 year olds and if they do they will be subject to a medical.

I suspect misunderstanding - Thai do not need medical insurance as government provides and cost would be excessive at older age (price does increase greatly).  Life insurance is also not common but local death insurance in amounts to cover funeral expense is common and often a village type system.  What is more common is that a family member will take physical care of elderly.