"Young men write letters."
That is ambiguous, I would say. "young men write letters" to mean that in general they do it would (in my opinion) better translate in : les hommes jeunes écrivent des lettres. Because the generality bears on "jeunes" (compared with old men who don't).
"young men write letters" to mean that a certain number of them do it (some), would translate in : de jeunes hommes écrivent des lettres". Then the rule of "des" replaced by "de" in front of an adjective would apply.