Most cruise lines charge full fare for first 2 passengers regardless of age, but I know that Royal caribbean have child and 3rd & 4th passenger reduced rates, so you can sometimes get good deals for having more than two in a cabin.Look on the website of the cruise company you are interested in.
Cruise Lines define the age of a child differently. Some are 11 yrs 12yrs 14yrs 18yrs. Some lines make no charge for kids at all times, some on special deals, so either trawl the 'net' to get the definitive or get advise from a good cruise specialist.
RCI is said to be best for kids. However MSC do free child places even for 14 year olds (although the air fare part wont be free). However, Brits will be in the minority and English speakers generally unless you go to the Caribbean with them, in which case there will be lots of American.
P&o offer free child places up to 16 on selected cruises well worth looking into but have to book early as places soon book up. 1 have one 14 one 5 so for me well worth it.hope this help.