Adult humans differ from one another in their individual inheritance, their personal biology, their specific life experiences, their unique perceptions, and their highly idiosyncratic interpretations of the world. Why, therefore, would anyone expect these diversely individualized beings, sovereign, willful, and living among tremendously varied circumstances, to somehow change (develop or progress) in lockstep, as a unified monolith, rather than as highly independent creatures each progressing, or not, at their own pace, in their own personalized way.
Not surprisingly, longitudinal studies (such as those chronicled by Dr. George Vaillant and others) show much more diversity than consistency among older adults — even among those of similar educational and socioeconomic backgrounds.
Any attempt to conveniently catalog changes in adulthood by defining and naming a stage or phase of adult development is fraught with so many caveats, exceptions, and exemptions as to render the schema largely impractical to irrelevant. This accounts, in part, for the lack of consistency in the findings for any researcher who has seriously looked at this question of adult developmental stages.
It is hard to ascertain and verify that which is not there.