How old is a child in Vietnam?

I have been learning a lot about Vietnam from Ben Pham, my new PhD student. It was her son's birthday on Sunday, and we have been discussing how to think about the age of children in Vietnam.
When Vietnamese parents say "my child is 4-years-old" they are using the Lunar calendar, where they begin counting on the date of conception (i.e., 9 months before the child is born).
When Vietnamese parents say "my child is 39-months-old" they are using the Western calendar, where they begin counting on the date of birth.
Therefore:

• 39-months-old = 4-years-old in Vietnam
• 48-months-old = 4-years-old in Australia

When working with Vietnamese children, it is important for Western people  to ask the age of the child in months!