Stewie Griffin is ageless, but his exact age remains a mystery. It's unknown how old Stewie entered existing. The baby persona from the animated television show "Family Guy" is immortal. His age is still determined.
Stewie, the clever little troublemaker, is impossible to determine 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 recent arrival? Only Stewie knows! This mysterious character continues to intrigue fans with his undisclosed age and youthful appearance.

List of Family Guy cast members Wikipedia