Youngster is a good, generic way to talk about a person who's younger than you are, especially a child. If you babysit a neighborhood kid, you can describe him as a youngster.
The noun youngster is a casual word meaning "child," and its synonyms include tyke and small fry. Older people sometimes use it to refer to adults who are younger than they are: "You youngsters can keep playing cards, but I'm going to bed!" Youngster has been around since the 1500's, when it replaced the earlier youngling.