So Nana and Grannie could be very old people (13)

I believe the answer is:

nonagenarians

'very old people' is the definition.
(a nonagenarian is 90 or older)

'so nana and grannie could be' is the wordplay.
'and' says to put letters next to each other.
'could be' indicates anagramming the letters.
'so'+'nana'+'grannie'='sonanagrannie'
'sonanagrannie' anagrammed gives 'NONAGENARIANS'.

(Another definition for nonagenarians that I've seen is " Very old people".)

I've seen this clue in The Times.

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