Stewie is a one-year-old with a very sophisticated attitude. He reached his first birthday in the season 1 episode "Chitty Chitty Death Bang", and has remained the same age ever since. His nature and mannerisms are juxtaposed with typical childlike interests and actions. While highly literate and able to cite pop culture references that date much further back than his age would let on, 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 the 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, a mind control device, a weather control device, robots, a time machine, a shrinking pod, and a teleportation device. Stewie employs these to cope with the stresses of infant life (such as teething pain, and eating hated broccoli) and to murder his mother Lois, with mixed success at best depending on the objective.
In other episodes, Stewie engages in other violent or criminal acts, including robbery, carjacking, loan sharking, forgery, and killing off many minor characters.
Stewie eventually realizes his dreams of matricide and world domination in the sixth season two-part episode "Stewie Kills Lois" and "Lois Kills Stewie". The events, however, are reverted in a deus ex machina ending, where most of the story turns out to be a computer simulation. Because of the rather disastrous ending for himself in the simulation, he decides to put aside his plans of matricide and world domination for the time being.
Despite his somewhat evil nature, Stewie does seem to have a softer side. For example, in "Chick Cancer", when Brian unwittingly makes a racist comment while the two of them are discussing Stewie's "marriage" to Olivia Fuller, Stewie is openly disgusted by it. When Brian profusely apologizes, Stewie leaves, saying, "You gotta work on that. Bad dog." When a cocaine-addicted Brian violently tells Megan to shut up over a trivial comment, Stewie counters with a supportive "Just because you can't feel your gums doesn't mean the girl can't feel your words". In one episode where Brian goes to find his mother and discovers that her dead body has been stuffed, Stewie says, "Someone must have said something funny, because your mother's in stitches!" After laughing for a few seconds, he tells Brian, "I'll leave you to grieve," before walking out of the room to give Brian some alone time with his mother. ~wikipedia