In October 2019, the Thai government officially made health insurance a mandatory requirement for long-term expats aged 50 and up.

The new rule states that those applying for or renewing a non-immigrant visa (O-A), which allows for a stay in the country for one year, will have to provide proof that they have health insurance.

The health insurance must cover at least 40,000 baht in outpatient coverage as well as 400,000 baht in in-patient care.

If a 50 or older expat is denied coverage by health insurers due to high health risks, he or she might still be able to obtain an O-A visa if he or she can show a high enough amount in their bank account.

For more information, we advise you to contact the immigration department in Hua Hin.

Source:
https://www.immigration.go.th/read?content_id=5d9c3b074d8a8f318362a8aa
http://www.thailawforum.com/blog/thailand-mandates-health-insurance-for-expats-50-and-up