Is there a word for people in the 80+ age group? I know octogenarian means 80-to-89-year-olds. Is there a word for people in their 80s, 90s, 100s, etc., inclusive?
For old in general, "superannuated." Otherwise, you'd have to go with "octogenarian, nonagenarian and centenarian," or possibly, "post-septuagenarian."
I suppose you're looking for a generic word like children; teenagers; youths; and the middle-aged. There is pensioners, a BrEng expression that includes anyone who is retired, which more or less corresponds to the over mid-sixties, but they include, ironically, people who are too young for your criterion.
Personally, I would classify a 75+ year-old, as being elderly. Someone approaching their 80s will be experiencing some of the symptoms associated with aging, and I have, in my mind, associated the elderly with fragility and vulnerability. I know this is not true for everyone, and I'm living in Italy where the average life expectancy is around 82 or 83 for women and a little less for men, but I think that we would agree that the majority of over 80-year-olds require some form of medical assistance.
To sum up, there isn't a term which accurately describes such a wide-ranging age group because they have little in common with one another. A 90-year-old is usually experiencing severe problems of mobility and probably experiencing some form of dementia too, so I wouldn't group them with octogenarians. And those "lucky" enough to past the 100 year-old-mark, generally speaking, have lost their independence and are being cared for (or should be) by family members or as guests in nursing homes.