Considering this is in 'Possesive' shouldn't it be 'Your age is ten years', rather then 'You are ten years old'? I know it doesn't sound natural in English, but it would make much more sense.
Yes, you're right.
The literal translation would make it more clear that a possessive is used in the Indonesian sentence.
"Umurmu" = "Your age"
"Umurmu sepuluh tahun." = "Your age is ten years"
However, I don't know if this literal English translation is accepted as a correct answer by Duolingo.