funny first dialogue with my Vietnamese classmates

They are two girls and four boys. The boys will study at another university, but were with the girls today, because their classes didn't yet start.
We didn't introduce each other, because we couldn't understand our names anyway (or know what word is a name and what word isn't). But generally it was very interesting because we carried out almost all our conversation in Chinese. It's good to talk to other students of the language, because they speak simply and slowly and are patient with me.
So one of them asked: "How old are you?"
"28" I said.
- "Oh, really, all of us are also that age."
- "hm."
- "Oh wait, did you say 28 or 18?"
- "I am 28."
- "Oh."
And they he said that they are all 18 years old. Wouldn't have been able to guess, since 28 year old Vietnamese people would probably not look much older than 18 anyway.
Later they asked me if I was married. Other culture, other focus...