It is up to the parent's discretion as to what age is right for their child to see their first movie. Children under 2, that are sitting in their parent's lap, do not require a ticket.
That differs from child to child. If they can sit for 90+ minutes quietly then you should be okay. I have taken children as young as 3 but I have also had a child I wouldn't take until 6 due to how difficult they found focusing.
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