This is the 10834th most frequent Polish word.
dziesięciolatek
A ten-year-old child.
Here, 'dziesięciolatek' is used as the subject of the sentence, referring to a ten-year-old child.
Dziesięciolatek biegał po boisku z kolegami.
The ten-year-old was running on the field with his friends.
In this sentence, 'dziesięciolatek' is in the instrumental case, following 'jest' to describe someone's age.
Mój młodszy brat jest dziesięciolatkiem.
My younger brother is a ten-year-old.
The word 'dziesięciolatek' is used in the accusative case as the direct object of the verb 'poznałam'.