Children

Children are simple. They act logically from their limited and often mistaken base of knowledge. If something is bad, it's because you've been told so. If someone does a bad thing, they must be bad. To children, the world is black-and-white with respect to ethics and morals. This makes for some interesting situations. In a recent episode of ER, and eight-year old shot his best friend. Why? He wouldn't agree to give up his shoes according to the bet they had made. And that "not fair."

Adolescents

Adolescents and teens are more strongly influenced by need than any other group. Like children, they tend to view the world more black-and-white than adults. Teens tend to combine the idealism of children with the rational faculties of well-developed adults.

The teenage years are those of raging hormones, though, and the stereotypical teen bases his or her decisions on the basis of attraction and repulsion. Witness the whole mess of teenage status and dating games.

Teens tend to justify their emotions, needs, and sense of justice through twisted logic. "The logic on your world does not resemble ours at all." -- Buffy to Xander, BtVS

Adults

Adults tend to decide things based on subtext. This is what confuses children, and to a lesser extent, adolescents so much, when dealing with them. Adults consider a host of factors when making a decision, such as social impact, moral standards, various relationships, and unspoken agendas as well as simple tactical advantage.

Still, adults by and large choose irrationally. The more intellectual types may try and reason things out; the truly wise among them simply choose based on experience; but for the most part, adults are almost as bound by emotional needs as teenagers are.

Other writing resources:

Awards Page
Character Breakdowns & Analysis
Fun With Profiling:Buffy the Vampire Slayer
How to Write
Motivations by Age Type
More on Schticks Page
My Attempt to Figure