"What is her age?"
Translation:Vad är hennes ålder?
The use of "sin" requires a subject (since it reflects back to the subject of the sentence). Therefore, you need "hennes" here.
Someone correct me if I am wrong, but I think that is like asking "What is her own age?" in Swedish. Doesn't really work.
"Her" is a possessive determiner, not a reflexive object. "Herself" in Swedish would be "sig själv", not "sin".
Hennes means her(s) as is used when she's the owner of something. ("Hennes ålder", "Bilen är hennes")
Henne means her as is used when she is the object of an action. ("Jag ser henne")
If you say han/hon/den/det/de and you're then referring back to the subject, it's sin/sitt/sina. If you're referring to someone else's things, it's hans/hennes/dess/deras
Han äter hans mat = He eats his (someone else's) food
Han äter sin mat = He east his (his own) food
Well, the rule is as I explained. If the subject is later also the owner of something, it's sin/sitt/sina. If it's someone else owning something, it's hans/hennes.
I'm just curious about how some Swedish questions are formed, I thought I understood but maybe not since I thought the correct translation would be " Vad ålder är hennes"?
Swedish doesn't use vad the way English uses what as in "what age", instead we use vilken/vilket/vilka. Secondly, asking "vilken ... är hennes?" is reserved for asking about objects ("which ... is hers?") rather than abstract things like age.
That said, a question can be formed with vad the way it's used above, like vad är hennes ålder? or vad är två plus två? for asking about a number.