I understand the Thai system. I've heard that other Asian cultures also apply a similar system, though I'm no authority on that. It makes sense. It's just as logical as the Western system to say "He's in his fifteenth year" meaning he's somewhere between his 14th and 15th birthday per the Western system.
What I've been meaning to ask for a long time but never had the opportunity, is this: If that is the Thai system, then wouldn't Thai law be interpreted in the same way? That is, if the law says a boy needs to be 20 to enter a bar, would that mean that the boy needs to be in his twentieth year (i.e. past his 19th birthday) to enter a bar?