A youthful figure (7)

I believe the answer is:

sixteen

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'a youthful figure' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see a link between them that I don't see?

This is all the clue.

Can you help me to learn more?

(Other definitions for sixteen that I've seen before include "Square number" , "Fourth power of two" , "Four squared" , "XVI" , "8 x 2" .)

I've seen this clue in the Evening Standard.

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