How old are people who are middle-aged?
The definition has shifted around as people live longer, and more importantly, as people object to being thought of as "old," and also as people are waiting longer to have kids, etc.
For me, 45-60 or so... not that I really plan to live to be 120.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
When Dante started his Divine Comedy with the line Nel mezzo del cammin' di nostra vita he meant he was 35 (presumably because of the Biblical limit rendered in the King James Bible as 'three score years and ten'); but as Barb's said, the date is in a constant state of flux. Medical, environmental, dietary, occupational, safety ....etc improvements keep raising the date when we're likely to die, so the middle goes up as well. Of course, usage isn't committed to using that precise mid-point, and I think people using the term today have in mind a range of ages from about 35 to 55.