Is a 40-year-old person too old to start an ATC career?
Some years ago (in the 80s) I tried out for ATC in Canada and took the preliminary screening test, which was a series of 25 diagrams representing radar displays with targets moving around different airways, crisscrossing each other in various directions, and you had to answer 2 questions for each diagram, whether or not targets at different speeds and altitudes would conflict with each other (there was a table with all the targets' speeds, tracks and altitudes).  It was a timed test and I wasn't paying attention so I was at question 45 when time was called and you had to drop your pencil right away.  So I didn't answer the last 5.   

I got one or two wrong, giving me a score of 43, a "pass" for the test, but there so many applicants they were only taking candidates with scores over 45 for the next stage.  Probably would have made it if I'd just answered the remaining 5 at random.

Anyway, actual background is not that important. They are looking for a very special set of mental skills, and someone has them or they don't.  Mainly, a very high cognitive and spatial IQ, and a very powerful memory.  To the extent that educational background matters, it would mostly be as a sign of intelligence.

I used to know a centre controller who told me that once trained and experienced, and having the required mental abilities in the first place, and the fact that the high stress periods don't last an entire shift (there are "rush hours" where things are intense, then relatively quiet) the job wasn't nearly as tough as the public perception.

She should just go ahead and apply and start the screening process, as I don't believe there are any specific professional or educational prerequisites, and there is no harm in trying.  She will probably find out pretty quickly whether there is a chance.  One thing going for her is the overall personnel supply situation which is affecting ATC to some degree as much as other parts of aviation (shortages of applicants with what it takes).