Detective Conan, who looks like a 6 year old boy, is actually a teenager of 16 years-old. In one of his cases, he was attacked and forced to drink a drug that made him shrunk and take the appearance of a little boy. He owns the most charming outfits, and a little boy being clever and resolving murders is highly enjoyable. No one knows he is actually Shinichi, the teenager, except his old neighbor and friend, who happens to be an inventor too. He has a girl friend who can kick ass (she is an expert in martial arts) but so far, she thinks he is only a cute lost boy.
This shounen has elements that make it fun to read: actual murders (blood included), likeable characters and an amusing MC. No wonder it is so popular.