Today, I read about secular acceleration in historical populations. It a bit complicated, but the short story is that the people in the past matured slower than we do today, due to nutrition and diseases.
Apparently, it was not unusual for males in the mid-19th century to get their voice break at 17 or 18 years old. OS being a child from a poor family would certainly have had a bad nutritional status, and therefore would mature rather slowly as well.
So at the beginning of Winnetou 1, you have to imagine him having his voice break. This is somehow an incredibly entertaining picture. (Luckily for Winnetou, he is two years older than Old Shatterhand, but would have been affected similarly).














