The definition of a tax payer in Thailand as it CURRENTLY stands.
1.Taxable Person
Taxpayers are classified into “resident” and “non-resident”. “Resident” means any person residing in Thailand for a period or periods aggregating more than 180 days in any tax (calendar) year. A resident of Thailand is liable to pay tax on income from sources in Thailand as well as on the portion of income from foreign sources that is brought into Thailand. A non-resident is, however, subject to tax only on income from sources in Thailand.
https://www.rd.go.th/english/6045.html
It's the same criteria that the US, UK and many other nations use to determine your income tax liability. It isn't your pension status, age, height or haircut that determines whether you owe the revenue department any money. It's the simple metric of how many days you are physically in the country. The US, UK and others determine this by having access to immigration databases (airport, airline, port and border passport desk data).