Zuckert, Catherine H., "Plato's Parmenides: Parmenides' Critique of Scrates and Plato's Critique of Parmenides," Plato's Philosophers: The Coherence of the Dialogues, Chicago, IL: University of Chicago Press, 2009, 147-79.

Excerpt:

The conversation depicted in the Parmenides between the elderly Eleatic and Socrates is usually thought to have occurred in 450. The Parmenides thus gives Plato’s readers their first view of the young Socrates, when he was eighteen or nineteen years old. This fact alone has produced a certain amount of scholarly controversy.

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