资料题：The main idea of the passage is that（　）。

How often one hears children wishing they were grown up, and old people wishing they were young again.Each age has its pleasure and its pains, and the happiest person is one who enjoys what each age gives him without wasting hie time in useless regrets.Childhood is a time when there are few responsibilities to make life difficult.If a child has good parents, he is fed, looked after and loved, whatever he may do.It is impossible that he will ever again in his life be given so much without having to do anything in return.In addition,life is always presenting new things to the child-things that have lost their interest for old people because they are too well-known.But a child has his pains:he is not so free to do what he wishes to do; he is continually being told not to do things, or being punished for what he has done wrong.When the young man starts to earn his living, he can no longer expect others to pay for his food, his clothes, and his room, but has to work if he wants to live comfortably.If he spends most of his time playing about in the way that he used to as a child, he will go hungry.And if he breaks the laws of society as he used to break the laws of his parents, he may go to prison.If however, he works hard, keeps out of trouble and has good health, he can have the great happiness of building up for himself his own position in society.