Rather than providing a poll with all the combinations as choices, let me just ask you to define the age limits of Middle Age as you see them. I’ll post periodic summaries (if needed) so the predominant one(s) may become apparent.
In my own estimation, I’d put the range at 35-60, with a fudge factor of a few years in either direction.
I would say middle age is around 45 to 60.
Unless you plan to live to be between 90 and 120 middle age is 40 to 50. A 60 year old has a life expectancy of 85 years. That puts their life at 70% over. Hard to call that the middle.
Middle age is 10 years older than me, at any point in time after I’ve turned 30. I’m 50 now. 
Life expectancy keeps creeping up. It’s now about 80 for a male in Canada. (Wiki page)
When I turned 40, 12 years ago, I did suddenly consider myself middle aged. I sure didn’t feel middle aged at 40, but the reality is that I was.
Heck I still don’t feel middle aged in a lot of ways.
ETA: To put a range to it, I’d say 40 to 60.
I’ve called years 30-60 middle-aged since I was in my 20s. I figure if you’re pretty lucky, you live 90 years. 30-30-30.
In my view, “middle” isn’t necessarily the 50% point. It’s a range on either side of that. If I go purely by feeling (as in health and mental capacity) I believe I passed out of youth (which started at maybe 18 with adolescence before that) around 30-35. I began “feeling” older when I got out of my 50’s. And I don’t feel old yet. I’m giving myself another five years before I’ll own up to being old.
Middle Age is the years during which you wish you had a spouse who was 15 years younger than you are.
If you know somebody twice your age, you’re middle-aged. If you only know people half again as old as you are, you’re old. Oldest person I know is 89, so, I’m old.
While multiples of 5 are nice roundish numbers, I’d say that for me, personally, “middle age” started around 37. I’ll have to let you know when it ends.
Medically, 65 is usually where we consider them “geriatric” in terms of elevated risks for falling and medication dosing and administration. So I guess they’d consider middle aged to extend to 64 and 364 days.
Nothing like “nailing it down!” 
But I often try to pin down “turning points” in history with that degree of precision. I may have even started a thread or two on the concept. As in: What day and hour marked the end of the Dark Ages?
Not sure when it ended, but it started in 410.
Middle age is 40 until you retire.
What we need is a term for age 30-40.
Yeah, though, but what specific date and hour?
Since I’m ignorant of the events of that year, what’s your basis? (Just curious, you know.)
Ignorance defeated once again. Thanks, and I’ll let the hour slip for now. 
This. For the same reason “Middle Class” doesn’t really mean “people who make within x standard deviations of the the mean household income”.
It’s “middle” as in you aren’t quite old as fuck yet, but you can’t reasonably be considered “young” by any stretch of the imagination. It’s also somewhat of a lifestyle definition as well. Like a middle age person should be at a certain point in their relationships, career, personal finances and whatnot.
The definition is made a bit more complex by the creation of a sort of “second adolescence” for people in their 20s (moving into early 30s now). So where 30 years ago, a guy in his late 20s or early 30s (maybe even younger if they started working right out of high school) could reasonably be expected to be a full grown adult, possibly with wife, kids, mortgage and a career, nowadays you see a lot of them living at home with their parents, struggling with careers and relationships and spending probably way too much time drinking, partying, playing videogames and otherwise acting like big teenagers.
Not that this prolonged adolescence is necessarily their fault. Economic troubles have made it more difficult for young people to get started. And we place more emphasis on education these days so many people are in their late 20s before they even start their career in earnest.
IOW, fewer people are having a mid-life crisis in their 30s and 40s because they are just recovering from their “quarter life crisis” in their 20s.
I’ve always felt like middle-aged should be defined as “old enough to have children in college” or “old enough to have young grandchildren.”
So 35 would be the absolute lowest end, but 40 more reasonable (i.e. you had a kid at 20, and they’re now 20 themselves). Then it extends to about 60 (if you had the kid at 40 and they were now 20), or perhaps slightly higher.
I like this definition a little better than strict age limits because it captures a lot of the life experiences that define your place in life.
I sort of figured I ought to feel middle aged when I was about 35, but I didn’t. By 40, I definitely started to feel it, though, and 45 is full-on middle-aged. Not sure when it ends and I roll over to “old.” I’m guessing late-fifties to sixty, maybe? Not geriatric, but edging over towards old…
When I was in mid-20s, I remember someone telling me I was middle-aged. I was totally WTF.
When I hear someone referred to as “middle-aged”, I picture someone in their late 40s. That’s when women start worrying about menopause, and maybe men start worrying about testosterone and receding hairlines? The 30s are wrought with age-related angst too, but it’s more like “I’m the same age as my mother when she had me but I haven’t done X, Y, or Z yet” rather than “I’m almost 50 and I haven’t done X, Y, or Z yet.” The latter seems more like a mid-life crisis thing.