Geriatric state that of the heir’s nearest sibling? (6,9)

I believe the answer is:

second childhood

I believe this clue is a double definition.

'geriatric state' is the first definition.
(geriatrics may experience a second childhood)

'that of the heir's nearest sibling?' is the second definition.
(an heir is typically someone's first child)

I've seen this clue in The Times.

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