Stewie is eternally young, but his precise age is shrouded in secrecy. It's unclear how old Stewie became on this planet. The infant persona from the animated television show "Family Guy" is immortal. His time on Earth is still discovered.
Stewie Griffin, the mischievous baby genius, is hiding his true age 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? Only Stewie knows! This mysterious character continues to captivate fans with his hidden age and forever young facade.

Les Griffin Wikipédia