Adults = Mature?

I have a question for all here. Human tend to get mature through the growing process, but why it seems that the more mature(or should I use the word 'age') they are, the longer they hold their grudge on, and they tend to remember all the bad things about the person they hate and what they have done to them. Sometimes the grudge is carried over from one generation to the next(just like what happen in some of the TVB dramas).

But, look at the children, they get along again in a short time after they fight or quarrel. No matter how serious the problem is, they tend to be friend to each other again. The time needed to get along again becomes longer as we get older.

So, why is there such difference exist? Is it the difference in the problem that they face? Is it adults are more mature and children are more innocent? Is it because the adults know more about the reality? Is it because adults feel less secured?

It is so rare to hear that children fight each other until serious injuries. But what about adults? Hurting each other with things that are able to hurt. From sharp weapons up to military weapons. Fighting with each other just to satisfy their own desire(territory, fame, wealth, etc), causing problems to all in the end. Adults can be consider to be more educated(religion teaching, knowledge, moral and ethics) than children, so why all these happens? Corruptions, crimes, wars and more. Why is that the history keep on repeating the same thing over and over again? Don't they feel bored with the same thing that repeat for thousand years?

In this case, should we use the word 'childish' on adults and 'mature' on children?