Your baby's age?

I asked a woman how old her son was and she said, "he's 37 weeks old." I said, "oh. 9 months, gotcha," then walked away. Really?  I am just curious to see how you ladies tell ppl how old your baby is? How do you do it? In weeks? In months? In years? I know people are different and have their own way of doing things, but that kind of threw me off. 

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