Family Guy has been on the air since 1999 and even if Stewie was a new born in the first season, he'd be at least 16 now.

So, here's Stewie, as he would look if he aged naturally. As I was drawing this I was imagining that Family Guy is basically Stewie's fantasy in which he's never grown up, and makes crazy inventions. When he gets alone time, he likes to get padded up, put on some overalls and snuggle with his old friend Rupert.

The hair style comes from a fantasy sequence where Stewie was imagining himself as a teenager.