1
general::
Distinguish between mature, experienced individuals and novices, as in The picket line will separate the men from the boys in the union. The idiom is used without respect to gender. [c. 1930]
American Heritage Idioms
2
general::
Phrase(s): separate the men from the boys [and] separate the sheep from the goats
Fig. to separate the competent from those who are less competent. (Not necessarily just about males.) • This is the kind of task that separates the men from the boys. • Working in a challenging place like this really separates the sheep from the goats.
McGrawhill's American Idioms And Phrasal Verbs