It takes three Sim days for the baby to turn into a child.
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Only the baby can grow up. Which is 3 days. The boy or girl Sim(child sim) can't grow up.
4 sim days
It usually takes three Sim days.
It takes about a week (in sim time)
If you are talking about The Sims 2, below the picture of the your Sim there will be a little bar, on the bar there will be how many days it will take until that Sim grows up.