The extreme coldness, and dull formality, which are pardoned in old age, make youth ridiculous. The levity, the carelessness, and the vanity, which are indulged in youth, render old age contemptible.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“The young, according to the common saying, are most agreeable when in their behaviour there is something of the manners of the old, and the old, when they retain something of the gaiety of the young. Either of them, however, may easily have too much of the manners of the other. The extreme coldness, and dull formality, which are pardoned in old age, make youth ridiculous. The levity, the carelessness, and the vanity, which are indulged in youth, render old age contemptible. The peculiar character and manners which we are led by custom to appropriate to each rank and profession, have sometimes perhaps, a propriety independent of custom; and are what we should approve of for their own sakes, if we took into consideration all the different circumstances which naturally affect those in each different state of life.” source