It might not have had much to do with Asian gangs though.   Traditionally, in many countries in Asia, people don't pay a lot of attention to birth dates.   I've known many people from China who had only a vague idea of the rough timing of their birth date.   In any event, people don't tend to get as wrapped up in birthdays as they do in Western countries where individualism is a more important feature of society.

So - perhaps not Asian gang related, unless there are links to western countries?