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On P&O, the age limit is 16. They have family saver fares. On some cruises children go free, although these are usually outside school holidays. Check P&O website
You need to check with each cruise line as they all have different rules on this. For example Royal Caribbean have reduced prices for 3rd / 4th passenger in cabin regardless of age. MSC Cruises count a child as up to 18 and all children cruise 'Free' with MSC if in same cabin as adults - you still have to pay the Air Fare, Transfer Fees and Port Taxes.