i v heard from a flying school lecture, from south africa.
came out of the airforce at about 20, got into the 707 and became a captain on that, and when 747 first came out, he was nominated as a captian at age around 25.
according to what he said, did all the conversion training and what not, simulator ride and stuff, and the first time he got on the real plane, it was the first time all the passengers got on as well, so everybody including the crews were going " well, what a big machine...."
unforunately he smoked a lot, had a heart attack at around 26, lost his medical, and never flew again.....