Here are three companies that accept up to 99 years old:
Chubb 26,500 baht for 1 year
https://thtravelinsured.chubbtravelinsurance.com/travel-insured/th-en/home.html
Luma Thailand Pass, by Tune Protect Thailand, a subsidiary of AirAsia, 31,500 baht for 1 year https://w3.tuneprotect.co.th/WebPortal/Pages/TAOnline01.aspx?Keys=BQSRGVKQQUKQAVY5P8KLG2KZ94ML26U3BJUXXGSF28UZG4RRX4JLAGFRW2
Thai General Insurance Association (mentioned already) 43,200 baht for 1 year
https://asq.in.th/thailand-covid-insurance