Yes but middle aged has always been a range, not a single age. And it makes sense to me for that range to have the average actual middle of life age (41, as you say) in the middle of it. Rather than as a starting point.
I don’t know why people get so defensive about it tbh.
ETA - having said that, I think there’s a reasonable argument to be made for just including adulthood. So adulthood 18-82, middle of that is 50. Therefore middle aged is 40-60