Old people sign (4)
I believe the answer is:
omen
'sign' is the definition.
(I've seen this before)
'old people' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'people' becomes 'men' (man is a kind of person).
'o'+'men'='OMEN'
(Other definitions for omen that I've seen before include "writing on the wall" , "Augury, portent" , "Event regarded as a prophetic sign of good or evil" , "Portent, prophetic sign" , "Portent of evil" .)