Stewart Gilligan Griffin is timeless, but his exact age is shrouded in secrecy. It's unknown how old Stewie became existing. The baby character from the animated television show "Family Guy" is forever young. His number of years is yet to be revealed.
Stewie Griffin, the mischievous baby genius, is hiding his true age from the world. Speculation runs rampant as to his actual age. Is he over a hundred years old? Or is he a recent arrival? The answer is known only to Stewie! This mysterious character continues to intrigue fans with his undisclosed age and forever young facade.

List of Family Guy cast members Wikipedia