Not quite as far back as Plato, but I was reading Seneca the other day who whines that physical education in Imperial Rome ain't what it used to be:

Can we possibly look on this as a liberal accomplishment for the youth of Rome, whom our ancestors trained to stand up straight and throw a javelin, to toss the caber, and manage a horse, and handle weapons? They never used to teach their children anything that could be learned in a reclining posture.