"Vad är hennes ålder?" sounds like a direct translation from English. As stated by Zmrzlina, it is more common to say "hur gammal är hon/han?" It is very rare to hear the example provided here
Yeah. If I heard someone ask what is her age, I would expect to hear "the stone age" as an answer.
In most varieties of English “How old is she?” would be much more usual, but in Irish English “What age is she?” is very common (in my experience).
Fun fact, Swedes typically don't state their age in years (i.e. I am 26 years years old), but rather by the year they were born. If a Swede asked my age, I would say that I was "born in '88."
I wouldn't say that we typically do it, but I notice a lot especially with young people.
Would that have anything to do with the ever-present person number? And I have to say it's creepy everybody and their grandmother can just see my age at a glance.
No, I don't think it has. However, it is convenient in a way, since you don't have to change it every year as you turn older, or even remember how old you are :-) . Lots of people find it unnatural, though, and you will never be wrong with "jag är X år gammal".
This's useful in fact, should I say that I'm 96 if somebody asks how old am I? :D
Is it the same thing as asking "how old is she"? Or will this sentece come further on?
"Hen" is also the gender neutral pronoun in Swedish, which apparently you won't learn in Duolingo.
Is "hen" used in everyday life? Or is it just used for political reasons by few peoples?
It's been gaining acceptance rapidly, and it's not uncommon by now. You'll find it in everyday speech, in newspapers, in legal text, in academia, and so on. But calling it common might be a stretch, and there are also lots of people who do not use it.
Sin has to refer back to a third person subject in the sentence, which there is none to be found here.
And why is it not "henne"? Because henne refers to someone earlier mentioned, which is not the case?
Exactly. her happens to have the same form in both cases in English, but if you compare with him instead it gets clearer: you wouldn't say him age.
henne is the object form of the pronoun hon but hennes is the possessive pronoun.
I have a really hard time remembering his vs. her. Any tips? Hennes and... what is the other one again? haha
Well, hans is also a male name ("Hans") so maybe you can connect it to that?