Senis mutare linguam
To teach an old man a new language

Age is slow at any kind of learning, but especially at learning a language, a faculty which is given by nature to children. It is generally well known that children can learn to speak any tongue, while older people do not achieve it or imitate it very badly. Hence the proverb 'to teach an old man a new language,' applied to those who labor at the wrong time and in vain. St. Jerome says in the preface to the four Gospels: 'It is a worthy task, but one of dangerous presumption, to judge others when one is open to be judged by all, and to teach an old man a new language, and turn back a world that is turning grey to its childish beginnings,' by which he means that the adult age is less amenable in all ways than those unformed and tender years, and Ovid puts this in an elegant metaphor:

The tree spreading wide its shade to the walkers beneath it
Once was a wand, when first it was planted in earth.
Then a mere hand's turn could pull it from shallow ground simply,
Now it stands firm, by its own strength grown huge.

For this reason the character must be formed while the age is malleable; the mind accustoimed to the best, while it is impressionable as wax. For later on, by the force of years, the mind becomes rigid, and we can hardly unlearn what we have learnt wrong; what we do not know can only be taught us with infinite trouble. I am not saying this to deter older people from learning, since it is never too late to learn, but in order to urge children to study.

We must not leave out the saying, common though it is, 'the old parrot takes no notice of that rod.' Although the meaning of the adage is not obscure, it will become clearer from the words of Apuleius, in the Florida, book 2, about the parrot: 'When being trained to imitate our country speech, it is tapped on the head with an iron key so that it really feels who is master. This takes the place of the cane while it is learning. The young bird learns at once, up to the age of two years, while the mouth is pliable and can be moulded, and the tongue is tender and vibrates quickly. But if an old one is caught, it is both unteachable and forgetful.' Thus far Apuleius. Pliny also mentions, book 10 chapter 42, the wonderful teachableness of the bird, but only up to two years old. Close to these remarks is a common but not inelegant expression: 'it is too late to accustom old dogs to the leash.'

from p. 200 in 1.