The -nya suffix means most of the time the possessive for the 3rd person. So it's "his" or "her".
Umur = a age/the age.
Umurnya = his age/her age.
But it can also means "the", depending the context.
Umurnya = his age/her age/the age.
Sometimes it's also the third person plural (their), but until now, I heard rather "mereka" than -nya to express "their".
But I don't know why the -nya added makes the sentence natural again. If someones knows, please tell us.
I don't think so.
Yes, the sentence maybe can used to interogative sentence, if you change point (.) to question mark (?) . And then, if you do it, the intonation must be change to question.
But, the sentence is statement, not question. Follow the mark on the after last word.
The sentence will changed to question if you add question word in the first word on the sentence, like "Apakah".
So if you add "apakah" the sentence will become "Apakah kamu berusia 10 tahun?" In English "Are you ten years old?"
Try to always write the full words, not the numbers. There's a rule on Duolingo, that they decided to accept only words, not number; And by the way the translation of "Umur kamu 10 tahun" is strictly "You are 10", here you have to make the (very small) effort to write the word.