If you entered Thailand as Visa Exempt or with a Tourist Visa, you can apply  for a Non-Immigrant Category "O" Visa at a Thai Immigration Office in Thailand provided you are age 50 or older, and meet the required financial requirements. 

Financial Requirements

Evidence of:

• At least 800,000 baht on deposit in a Thai bank
  

• A monthly income equivalent to at least 65,000 baht per month