Is Your Character an Adult or Child (not age)?:
Try a simple experiment. Think about a person you know and blurt out, without too much thinking, either "adult" or "child." Now try another, and another. We're not talking about their age here. There are definitely older people who can be labeled a child, and vice versa. Children are given to playfulness, extraversion, openness to experience, and having a silly sense of humor. While an adult personality may be more reserved, task focused, and, although they like a good laugh, are more interested in getting back to the serious issues at hand. Child personalities are more likely to make mistakes and learn from them. While an adult is more likely to shrug off a mistake as not their fault. When creating a character, it's very important to decide which of the two major personalities she will fall in to.
In general, heroes are children while villains are adult personalities.
Children are more open to change. They know they don't understand everything and are more willing to learn as they go than a more staid adult type. Adults tend to think they know everything since they've experienced more. As a result, change can come about more slowly in an adult personality. The childlike hero is able to adapt to the situation, which can be the key to winning the story struggle.
The child personality tends to live in the moment, never really planning in advance, giving her a talent for thinking on the spot and coming up with an innovative way out of a situation. Adult personalities are more likely to sit back and think things through, wasting valuable time and resources. They have plans (sometimes dastardly plans) and when things go wrong, they may stick with the plans rather than "winging it." This can be their undoing.
A child personality is more physical, able to run and catch that swinging cable, than adult types who tend to get others to do it for them. A child-hero is more likely to run into the burning house and save the goldfish than an adult-villain. Why get the suit dirty when there are firefighters trained for that sort of thing?
Adults are careful speakers and exhibit more advanced language skills than the child type. A child may find his mouth getting him into more trouble, while the adult relies on speech to make herself understood.
The child personality is more outwardly emotional, screaming, crying, and fighting, while the adult is more calm and self-controlled.
Again, it's important to point out we're not talking about age. You could have a scene on a playground with eight year olds and make it clear who are the adults and who are the children personalities. This technique can prove to be an effective tool in differentiating your characters and giving them instant life and theme.