That time of life when you hearten mother? (5)

I believe the answer is:

youth

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'that time of life' is the definition.
(youth is a kind of time of life)

'you hearten mother?' is the wordplay.
'hearten mother?' becomes 'th' (I can't explain this - if you can you should give a lot more credence to this answer).
'you'+'th'='YOUTH'

'when' acts as a link.

Can you help me to learn more?

(Other definitions for youth that I've seen before include "Young person" , "7" , "Juvenile" , "Inexperience" , "Immaturity" .)

I've seen this clue in the Evening Standard.

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