Stewie Griffin is eternally young, but his exact age is shrouded in secrecy. It's unknown how old Stewie has been on this planet. The toddler persona from the animated television show "Family Guy" is forever young. His number of years is still determined.
Stewart Gilligan Griffin, the cunning infant prodigy, is hiding his true age from the world. Speculation abounds as to his actual age. Is he a century old? Or is he a recent arrival? The answer is known only to Stewie! This enchanting character continues to mesmerize fans with his hidden age and youthful appearance.

Stewie Griffin Wikipedia