This is the 4021st most frequent Swedish word.
nioåring
A nine-year-old.
The word 'nioåring' here is used as a noun referring to a nine-year-old person, specifically the speaker's brother.
Min bror är en nioåring som tycker om att spela fotboll.
My brother is a nine-year-old who enjoys playing soccer.
In this sentence, 'nioåring' is functioning as a noun in a prepositional phrase indicating age during a time period.
Som nioåring var hon redan duktig på att lösa matematiska problem.
As a nine-year-old, she was already skilled at solving mathematical problems.
The use of 'nioåringen' in this sentence is as a definitive noun referring to a specific person who is nine years old.