Stewie is a one-year-old prodigy who has a very sophisticated psyche, is able to speak fluently, and has a peculiar English accent. He reached his first birthday in the season one episode "Chitty Chitty Death Bang," and has not been referred to as being more than a year old since, although it is presumed that he has aged to the age of at least three, as he is seen in several episodes going to Pre-school. While highly literate and able to cite pop culture references that date to long before his birth, he is also entranced by Raffi and the Teletubbies. Stewie succumbs to other weaknesses of children his age – he believes Peter has truly disappeared in a game of Peek-a-Boo, talks to his teddy bear (Rupert) as if he were alive, is overcome with laughter when Lois blows on his stomach, and has no idea how to use a toilet. MacFarlane has stated that Stewie is meant to represent the general helplessness of an infant through the eyes of an adult. Per cartoon physics, his ability to move objects of greater weight than himself is not surprising to other characters, nor is his ability to retrieve firearms from hammerspace.
Stewie's mastery of physics and mechanical engineering are at a level of science fiction. He has constructed advanced fighter-jets, mind control devices, a weather control device, a teleportation device, robots, clones, a working Transporter device from Star Trek, time machines, a Multiverse Transporter, a shrinking pod, as well as an assortment of guns including lasers, rocket launchers, and crossbows. Stewie employs these to cope with the stresses of infant life (such as teething pain, and eating broccoli) and to murder his mother, Lois, with mixed success at best depending on the objective. As made clear in the pilot episode, Stewie's matricidal tendencies are a result of Lois constantly (and unwittingly) thwarting his schemes, and so he desires to kill her to carry out his plans without her interference.
In other, recent episodes, Stewie engages in other violent or criminal acts, including robbery, owning firearms while underage, carjacking, loan sharking, forgery, and killing off many minor characters (with a tank, guns, and other assorted weaponry).