Keira Metz

A very specific question, I know, but I was wondering for something I'm working on. Keira Metz has always struck me as one of the younger sorceresses from what we see of her in the books and third game, but younger can mean anything from 40-100 years in terms of mages. Keira doesn't strike me as one of the really old mages like Tissaia de Vries, though.

She's definitely younger than Triss as it's stated that Triss is the youngest member of the Lodge of Sorceresses, but that isn't too helpful as we don't know Triss' exact age. Do you think Keira Metz was already around and a sorceress in the Royal Council before Foltest was king?

Can a King have more than one mage/witch in their court in a time? When Geralt first met Triss, she was the advisor to King Foltest. And Geralt also mentions that Keira was also an advisor to Foltest. So was there overlap? Or do court assignments really change so easily?

It's possible I just missed that part in the books. But any information would be helpful!