The 8th of March is International Women's Day. Chinese students have long designated 7th March as Girls' Day. The thinking is that a girl may be only one day away from becoming a woman.
In the expression to separate the men from the boys it is important to remember that a boy may be only one day away from becoming a man. Every man was once a boy. Every boy, regardless of his personal qualities, merit or competence, will become a man. It is not the case that only the strongest, best or most macho boys turn into men. All boys become men.
Therefor I feel that ideas such as wheat and chaff. sheep and goats, etc., miss the point that a boy and a man are intrinsically identical in all essentials, differing only in how far along they are in life.
A particular man is not superior, as a being, to a particular boy. When the boy is older he may be a better man or a worse one (however we define a good man). When the man was younger he may have been a better or worse boy.
I also do not like words like kids or children or lambs in this context. A boy may be an older teenager, on the verge of manhood, no mere kid.
Suggestions that the concept behind the phrase is not effective as a means of promoting learning may be right, but that is a separate matter to how the phrase might be degendered. I like suggestions re apprentice and master, or rookie and veteran, although master is masculine (to me) and I wrongly associate rookies and veterans as male.
So I am going to suggest "here" and "near" as distinguishing between those who have already reached a particular standard, and those who may be as little as one day away from reaching it.
The ability to do proofs of this type is what separates the here from the near in this class."
However this doesn't fit well with the second example so perhaps "really" and "nearly" might work.
It's the training and experience with the specific disability that's going to separate the really from the nearly."