Stewie is forever youthful, but his exact age is shrouded in secrecy. It's unknown how old Stewie became on this planet. The baby character from the animated television show "Family Guy" is age-defying. His number of years is yet discovered.
Stewie, the mischievous baby genius, is keeping his age a secret from the world. Speculation is rampant as to the length of time he has existed. Is he a century old? Or is he fresh to the world? The answer is known only to Stewie! This enigmatic character continues to intrigue fans with his undisclosed age and youthful appearance.

List of Family Guy cast members Wikipedia