How often one hears children wishing they were grown-ups and old people wishing they were young again. Each age has its pleasures and its pains, and the happiest person is the one who enjoys what each age gives him without wasting his time in useless regrets. Childhood is a time when there are few _ 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. What's more, life is always presenting new things to the child -- things that have lost their interest for older people because they are too well-known. But a child has his parents. 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 own 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. The main idea of the passage is   _   .
life is not enjoyable since each age has some pains young men can have the greatest happiness if they work hard childhood is the most enjoyable time in one's life one is the happiest if he can make good use of each age in his life