Why book of noblemen won't include those too old? (8)

I believe the answer is:

baronage

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'why' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I don't understand how the rest of the clue works.

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(Other definitions for baronage that I've seen before include "Body of lowest-ranking nobility" , "Group of nobles" , "Nobles collectively" , "Some of the aristocracy" , "Lords" .)

I've seen this clue in The Irish News.

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