I am not aware that this has changed... So, they can work in a bar at age 18 - but cannot drink until they are 20!
LABOUR PROTECTION ACT B.E. 2541 (1998)
Section 50
An Employer shall not require a young worker under eighteen years of age to work in the following places:
a slaughter house;
a gambling place;
a place for dancing, Ramwong or Rong Ngeng;
a place of selling and serving food, liquor, tea or other beverages with a pleasing person to treat customers, or with a place for relaxing, or massage service for customers; or
Other places as prescribed in the Ministerial Regulations.