Native Americans' old son gets older (6)
I believe the answer is:
osages
'native americans' old son gets older' is the definition.
I can't tell whether this defines the answer.
'old son gets older' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'son' becomes 's' (genealogical abbreviation for son).
'gets older' becomes 'ages' ('age' can be a synonym of 'get older').
'o'+'s'+'ages'='OSAGES'
Can you help me to learn more?
(Another definition for osages that I've seen is " American Indians".)