They are in fact teenagers by the modern definitions. You were a full responsible adult shortly after puberty. Many of the greats did their best work before 20.
Who did their best work before 20? I looked up a bunch of Greek philosophers, and it doesn't seem to apply to any of them. Socrates was in the military during his early life, and Aristotle did his most famous work late in life.
One possibility is that GP is thinking that "life expectancy was 25, so 20 means you're an old geezer on the deathbed." Alternatively, it may also be a reference to the extreme youth of Alexander the Great, who became king of Macedon at age 20 and proceeded to put down a Greek rebellion and then conquer Thrace, Persia, Egypt, Bactria, cross the Hindu Kush, conquer bits of India (then unknown to the Greeks) and was only stopped from going further when his army said "it's been long enough, can we go home?" He died 3 years later, at age 32. Obviously, he is very much an outlier.
Aqwis|9 years ago
jcranmer|9 years ago
ajcarpy2005|9 years ago