Letters 118.14

Surely because of its magnitude. It is no new idea that certain objects change as they grow. A person, once a child, becomes a youth; his peculiar quality is transformed; for the child could not reason, but the youth possesses reason. Certain things not only grow in size as they develop, but grow into something else.

Seneca, Letters to Lucilius 118.14, translated by Richard Mott Gummere