Notedly advance age (3)

I believe the answer is:

eon

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'age' is the definition.
(eon is a kind of age)

I can't explain the remainder of the clue.

(Other definitions for eon that I've seen before include "Prolonged period" , "Immense period" , "In time, long division" , "thousand million years" , "Vast age; eternity" .)

I've seen this clue in the Evening Standard.

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