Here's from the Princess website:
Age requirements
Guests under the age of 21 years must travel in a stateroom with a guest 21 years or older who shall assume responsibility for their care during the cruise. For family groups booking multiple staterooms, the minimum age for each stateroom is 16 years of age, provided they are traveling with a parent or legal guardian. We are unable to accept group reservations for student or youth groups that do not conform to our minimum age requirements. Each guest agrees and warrants that he/she will supervise any guest in his/her care at all times to ensure all policies, along with all other rules of the Carrier and ship, are strictly adhered to by all guests under their supervision.
https://www.princess.com/en-us/faq/pre-cruise#prepare-for-your-cruise