It’s habitual for us to get older (5)
I believe the answer is:
usage
'it's habitual' is the definition.
I can't tell whether this defines the answer.
'us to get older' is the wordplay.
'to get older' becomes 'age' ('age' can be a synonym of 'get older').
'us'+'age'='USAGE'
'for' is the link.
Can you help me to learn more?
(Other definitions for usage that I've seen before include "Tradition - rate of consumption" , "Way of employing" , "Habitual custom" , "Common practice" , "etiquette" .)