Old people finally cast in films (5)

I believe the answer is:

picts

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'old people' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'finally cast in films' is the wordplay.
'finally' says to take the final letters.
'in' indicates putting letters inside.
'films' becomes 'pics'.
The last letter of 'cast' is 't'.
't' put into 'pics' is 'PICTS'.

Can you help me to learn more?

(Other definitions for picts that I've seen before include "People living in Scotland in Roman times" , "Ancient Northerners" .)

I've seen this clue in The Daily Express.

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