I just pulled up the parental consent form here where I work. It is a generic form accepted by all the cruise lines (so individual requirements for age mey vary between lines). It says for anyone under the age of 21.
The letter or form must not only be signed by a parent but must be signed by both birth parents listed on the birth cert. Even if one parent has sole custody. In the event of divorce both must sign allowing you to take the child out of the country. It must also be notorized. The only exception is in the event of the death of one of the parents and in that case a death cert must accompany the letter/form.
They have not been enforcing this rule as much lately as they did at one time. However if another high profile case hits the news about a parent (or anyone else) fleaing to another country with a child during a custody battle they'll be checking for it again for all children boarding.
Be on the safe side and get the letter or form.