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English
How old are you? (universal)
Chinese
你多大了?
Pinyin
nǐ duō dà le
Literal meaning:
You How (Many) Big [Grammar particle]
The Chinese character 多 (duō) can mean "many" but when used in this context is used to make the sentence a question about the extent of 'big' that the other person is.
This phrase may be better used when asking kids their age. If you ask a question where the answer is a number between 1-10, then you should use the word 几 (jǐ), but if you anticipate the answer to be bigger than 10 then you should use the word 多少 (duōshǎo). But what if you think a kid is between the age 9 and 11? Well, then this phrase may come in handy, as it doesn't reveal what answer you expect.
Is it important? Well, that depends on the situation.