For those who dont want to be "middle aged"
yet one really imaginative definition I can think of could be defended by science. A few years back I read two books on modern aging science. In them I was surprised to find that some modern theories hold that people do not 'age' in a scientific sense until the age of 16-18, prior to that they 'grow' and stave off accumulation of senescent traits (aging) by biological design. Biological immortality may vanish soon after a cell begins dividing but the term 'aging' could be reserved for degenerative phases of life.
Thus- if aging starts after, say, 16.. and people can conceivably live to 122.. the half way mark between those is 69. So people within several years of that could still defensibly call themselves middle aged.
Then again if you want to be unpopular with the fairer sex, one could argue women living in a community dynamic become biologically invisible after menopause (because they are no longer part of the continuation of the species) so middle aged would be 20-30. Be ready to duck if you say that.
Over all I think all-encompassing definitions are nothing more than a way to amuse oneself. Whats middle age for an actual living individual depends on that individual and how they live their life. I've known people that rode themselves so hard their health was showing rapid deterioration by their mid-to-late 20's while others kept up good habits and were still in good health and filled with stamina and adaptability at 50.
With health and vigor in your pocket, definitions of phrases shouldnt worry one too much.