At what age do you have to pay for an infant?
NCL Dream!

On NCL kids under two travel at a substantially lower rate.

It can range from only taxes to a couple hundred dollars. It depends on the sailing and ship sometimes. Check it out with your travel agent.

Send them to grandmas, cheapest fare you can get.

Are you going on Norwegian Dream? If so, that's a great ship when traveling with an infant as the cabins are relatively large. A pack-n-play fits with plenty of room to spare. (Spoken through experience.)

Yes, NCL offers deeply discounted rates for infants, as noted by Cruise Fanatic and gocruiser.