Q. I've always heard that a child's character and personality are pretty well set by the time he/she is 5 or 6 years old, and that any dysfunctional behavior patterns the child still exhibits at that age are going to be almost impossible to change. Is that right?
Your question reminded me of the adage, “You can't teach an old dog new tricks.” That's not true.
It's quite possible to teach an old dog, or in this case an older child, new tricks, new behaviors. In both cases, however, the older the dog or child, the more difficult it's going to be for either to unlearn old behavior patterns.
That's why I so frequently stress the importance of establishing functional disciplinary foundations by age 3. The earlier those disciplinary understandings are in place, the better for child and parent. Conversely, the longer misbehavior is allowed to develop, the more difficult it's going to be to correct.
Temperament and personality are interchangeable terms referring to general attributes like extroversion versus introversion. Most researchers feel those qualities are inherent and relatively fixed, but keep in mind that humans adapt to new circumstances and contingencies more easily and successfully than any other macroscopic species.
For example, an introvert can learn to be more outgoing, but he will probably always say that he's fundamentally shy.
The bottom line is that neither personality nor behavior is fixed by age 5 or 6, but modifying the former will require cooperation on the part of the child as well as a good deal more of everyone's perseverance and patience.