For someone under 18 to drink alcohol in licensed premises, except where the child is 16 or 17 years old and accompanied by an adult. In this case it is legal for them to drink, but not buy, beer, wine and cider with a table meal.
Drinking Laws in Thailand: A few facts. The drinking age in Thailand is 20. It is in the interest of bar owners to enforce this as establishments do occasionally get raided for underage drinkers and patrons under the influence of illegal substances. This doesn't mean you can't buy alcohol if you are underage.