the end of the saat season i think... part of stewie griffin - the untold story, he meets his older self in part 2, then anda get to know him in part 3, where anda find out meg has a sex change, chris gets married, stewies a virgin and lois is still alive :)
Its three different episodes in season four, titled: "Stewie B. Goode," "Bango Was His Name, Oh!," and 'Stu and Stewie's Excellent Adventure." Its also in the movie "Stewie Griffin: The Untold Story"