Good fellow - who is this?

"Good", including "good fellow" means - a young man in full force dawn
And it's not about Cartlon from the cartoon. In the old days, life expectancy was significantly less. After forty years old age came. Conditions of life were severe, the body quickly worn out. The girls who reached the age of 12 and the boys who reached the age of 15 were considered to be adults.

It was at this age that marriages took place. Peter I by his decree raised the age of brides to 17 years. But in fact it has not been performed for more than one century.
Physical work, military training began with a young age. You do not need to think that your children were treated badly. No, they were loved and protected. But the fact of the matter is that without certain skills that were instilled from early childhood, it was simply impossible to survive.

https://tehnika.spb.su/kartinka-ivan-carevich-i-lyagushka.html
Boys by the age of 15 were already stately, handsome men in their prime. The expression "good fellow" means:

• a well-built young man;
• a guy who is perfectly physically prepared (in sports, martial arts, etc.).
  Remember the expression "good fellow and red maiden"? - "Well built, young man and beautiful face girl."

In this phraseology, the word "good" characterizes the physical, and not psychological, qualities of a young person.