A Goodreads user asked:
How old was Margaret Lea when She first met Vida Winter?
Rebekah She mentions the current time as "30 years later" after she learned to read, so I'm guessing mid 30s.
Dalia It doesn't say but you can guess that she might be in her late twenties or early thirties. She was not married and still living with her parents. Vida also calls her "child" when she gets close or emotional with her.