Plutarch, On the Education of Children 9

from http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Moralia/De_liberis_educandis*.html

For just as those who have been in fetters for a long time, even if later they be set free, yet, because of the long-continued habituation to their bonds, are not able to walk freely, and are not sure on their feet, so is it with those who for a long time have kept their speech under close restraint: if ever it becomes necessary to speak offhand, they nevertheless keep to the same type of expression as before. But to allow those who are still young to speak extempore stands responsible for the worst sort of rambling talk.