Household in which people grow old (6)
I believe the answer is:
'household' is the definition.
(eg menage a trois, literally a household of three)
'people grow old' is the wordplay.
'people' becomes 'men' (men are people).
'grow old' becomes 'age' (synonyms).
'in which' is the link.
(Other definitions for menage that I've seen before include "Social unit living together" , "live together" , "Persons of household" , "a household, may be naughtily a trois" , "Members of a household" .)