I know several of them, but in not one case is that the name on the ID card.

 

These working Thai boys cover their tracks with stage names, so it must be OK to use them. 

Rather like most of us board participants use handles not related to our real names. 

 

The boys use nicknames ชื่อเล่น chêu lên (actually every Thai I know uses a nickname, except for doctors and professors at my former workplace) which are not in their ID card. Real names are longer and can be difficult to write, have a look at taxi driver's name card (behind the LED sign) when you sit in the front seat.

He is 20 (almost 21). But he says when you ask for his age that he is 21. Many boys add 1 year for some reason.

 

 

I notice more boys do this. i think in Thailand when you are say 30 you will say 31 because you are in your 31st year. So a boy who turns 18 today will say from today on that he is 19, because his 19th year has started. I think it is something Thai.

 

while abidismali's explanation makes some sense, too, I think it is much simpler: the simply subtract the year they were born in (say, 1990 / 2533) from the current year (2015 / 2558) and thus arrive - mathematically correct - at their age in numbers of years (25). This obviously disregards the months, but is a simple and not necessarily incorrect way to calculate age measured in years. Thais have a penchant for simplicity ;-)