Having now read the books (twice), I believe the answer is "no, they don't age, but film is an imperfect medium."
Marthe was indeed turned late in life, so she's supposed to look like she's in her 60s or 70s. That's easy.
With Ysabeau, it was the proverbial "pick any two" dilemma: gorgeous, young, and terrifying don't generally coexist in human females, and since vampires don't tend to show up to casting calls, well, the casting director in this case (apparently with the author's full blessing) chose gorgeous and terrifying and ran with it.
As with all casting questions, it helps to keep in mind that the role of Juliet was originally played by a young boy.
Editing to add: I found an interview with Deborah Harkness where she basically says the same thing:
And the other thing is like really we only have human beings to capture these characters. [...] Remember it’s an adaptation. Remember sort of like the great Mart, Martha, Marta comment, that there’s kind of no right way to do any of this, because it is an interpretation, just like you could go see a Shakespeare play and everybody’s wearing flower costumes. Did Shakespeare ever imagine that? Hell no..
("the great Mart, Martha, Marta comment" refers to an earlier question in the same interview about the correct pronunciation of Marthe.)