I think the more natural way to say this in English would be "...since she turned forty," but then we would probably get all confused about translating blev as turned. Okay. Never mind. :-)
You're right. It should be "turned forty". I'm not sure that "became forty" is the sentence it's meant to be. "became forty" means she turned into forty somethings.
Whats the difference between "kvinden" and "damen"?
The woman. The Lady.