Stewart Gilligan Griffin is timeless, but his specific age is shrouded in secrecy. It's unknown how old Stewie has been existing. The baby persona from the animated television show "Family Guy" is forever young. His age is still revealed.
Stewie Griffin, the mischievous baby genius, is keeping his age a secret from the world. Speculation is rampant as to the length of time he has existed. Is he over a hundred years old? Or is he a newcomer? The answer is known only to Stewie! This enigmatic character continues to captivate fans with his undisclosed age and forever young facade.

Chris Griffin Wikipedia