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The 12 month rule is when parents have to stop saying how many months old their baby is after they reach 1 year old. Some parents say their baby is 34 months old. They make you do extra math when they could easily say 2 years and 10 months old.
Woman 1: "Aww how old is he?"

Woman 2: "26 months old"

Woman 1: "Don't say 26 months old it breaking the 12 month rule."
by ImmediateAce September 17, 2013
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