Middle-aged ones cram classes (6)
I believe the answer is:
genera
'classes' is the definition.
(more than one genus - a class or category)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for genera that I've seen before include "similar groups" , "Biological groups" , "Orders with common characteristics" , "Categories making up a family" , "Taxonomic categories (biology)" .)