Jack Frost's age varies depending on the source:

• In the Rise of the Guardians film, Jack Frost is over 300 years old but eternally appears as a 17-year-old teenager. He is immortal and stuck at this physical age, though he can slightly change his appearance within limits

• William Joyce, the creator who contributed to the books related to Rise of the Guardians , prefers Jack Frost to be stuck at the age of 14, reflecting a younger, eternally adolescent character

• DreamWorks, the studio behind the Rise of the Guardians movie, chose to depict Jack Frost as 17 or 18 years old to make his story more plausible and relatable as a young adult rather than a child

In summary, Jack Frost is traditionally depicted as an immortal being who has been around for centuries but appears perpetually as a teenager, either 14 years old according to the original books or 17-18 years old as per the movie adaptation