Young Australian? He must be about 70! (4,6)
I believe the answer is:
baby boomer
'he must be about 70' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'young australian?' is the wordplay.
'young' becomes 'baby' (baby is a kind of young).
'australian?' becomes 'boomer' (boomer can mean a large kangaroo).
'baby'+'boomer'='BABY-BOOMER'
Can you help me to learn more?
(Other definitions for baby boomer that I've seen before include "born just after the war?" , "Person born shortly after WWII" , "one arriving in a crowd" , "Joey" , "Someone born in the decade after WWII" .)