Hellboy ages very differently from humans. In the story "Pancakes" he is two years old but appears to be somewhere between 6 and 10 human years old. In "Nature of the Beast", set in 1954, the ten-year-old Hellboy appears fully grown. His rapid physical maturation is in contrast to his actual rate of aging, however, which seems to be much slower than humans. Throughout the sixty-year span of time depicted in the comics, he does not age beyond the plateau of physical maturity. This mystical aging process is similar to the other demons and supernatural beings that populate Hellboy's world. The lifespan of a demon or half-demon as Hellboy's mother was human, are left undefined within the comics and seem to range from decades to many thousands of years. In the movies, Hellboy's aging process is described by BRPD as "reverse dog years".