"Скільки їй років?"

Translation: How old is she?

[heathmar]

If one was accidentally to use 'вона' here when speaking, would it be a serious deal or just an odd look?

[skstudio]

It would be just grammatically wrong and no more. That is, you cannot insult a person with this or something like that.

[normanls]

I've used "її" instead of "їй" are these datives (not) interchangeable ?

[matiasluis2077]

Її is the possesive form of her

[ScottWilli909480]

So вона and їй can be freely interchanged or a set of rules exist?!!!