Stewie Griffin is eternally young, but his exact age is shrouded in secrecy. It's unclear the exact number of years Stewie became living. The baby persona from the animated television show "Family Guy" is age-defying. His number of years is still revealed.
Stewart Gilligan Griffin, the cunning infant prodigy, is impossible to determine from the world. Speculation abounds 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 hidden age and forever young facade.

Griffin family Wikipedia