Short answer,Short answer: not defined yet.
Long answer:
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 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 (Hellboy's mother was supposedly a human witch) is left undefined within the comics, and seems to range from decades to many thousands of years. (source)