Let's see, if an average life span is 90, (and it may even be lower,) and one divided that into three parts, that would mean until the age of 29, one is "young", then from 30 to 59, one would be "middle aged", and from 60 on, one would be "old." Since when does middle age mean only one year, like 40. With that kind of thinking, 39 and younger is "young", and 41 and older is "old." We all know that's a bumch of crap. The word middle clearly suggests dividing into three equal sections. The math really is quite easy. So for all of you in your 30s, deal with the reality, you are, in fact, already middle aged. Yeah, I know it's tough, but it's reality.
Different organization try to define it in their own way, but I think that simple math works the best.