Wasn't František married? Maybe that's why they have so many problems, he's still a boy, hahahaha
There's a lot of strange stuff going on with these people... :-)
He's probably a metaphorical boy, a man child - that's why his wife cries when he's asleep :/
When child marriage does occur, it's usually with the genders the other way around. Good to know Duolingo is welcoming such diversity!
Why while is not accepted
"František is a boy, while Matěj is a man" is an accepted translation. However, there is a report in the system that came in at the same time as your comment. The reported sentence was "Frantisek is s boy while frantisek is a man." Oops?
That might be explained by above comment from tomasmarc!