Stewart Gilligan Griffin is timeless, but his exact age is shrouded in secrecy. It's unclear the exact number of years Stewie has been living. The baby character from the animated television show "Family Guy" is immortal. His time on Earth is still revealed.
Stewart Gilligan Griffin, the clever little troublemaker, is keeping his age a secret from the world. Speculation abounds as to how many years he has been alive. Is he a century old? Or is he fresh to the world? The answer is known only to Stewie! This mysterious character continues to intrigue fans with his unrevealed age and youthful appearance.

Griffin family Wikipedia