Norwegian Cruise Line has announced that it will remove most masking requirements for all sailings from 1 March 2022.
And in another change to their policy is that a certain number of unvaccinated children under the age of 5 will be able to sail. Previously regardless of age, all passengers had to be vaccinated.

From March 2022 all aged 5 and over need to be vaccinated but 4 and under do not. However, NCL has said that the under 5’s will not make up more than 5% of passengers on board.
NCL Mask Requirements
As you can see from this announcement below from the NCL website masks are still recommended, but will not be compulsory.

The decision about whether to wear a mask or not will be left up to each individual guest.
NCL Covid Testing Requirements

This is changing too from March 1 2022.
Prior to March NCL has been testing passengers at the embarkation port and has been covering the expense of this. The initial test is an antigen test, which if it is positive will be confirmed with a PCR.
However, from March 1, 2022, NCL passengers will be required to provide proof of a negative COVID-19 antigen or PCR test result administered by a verified third party or via a medically supervised home test.
This test must be within two days prior to boarding for cruises originating in a U.S. port and three days prior for voyages departing from a non-U.S. port.