Stewie Griffin is ageless, but his precise age is shrouded in secrecy. It's unclear the specific duration of time Stewie became existing. The infant persona from the animated television show "Family Guy" is forever young. His number of years is yet revealed.
Stewart Gilligan Griffin, the cunning infant prodigy, is keeping his age a secret from the world. Speculation abounds as to the length of time he has existed. Is he ancient beyond measure? Or is he fresh to the world? The truth lies only with Stewie! This enchanting character continues to mesmerize fans with his undisclosed age and forever young facade.

Griffin family Wikipedia