Someone who's octogenarian is older than 80 and younger than 90. If your great-aunt is 84, she's an octogenarian.
You can use this word as an adjective or a noun for anyone who is in their eighties. Many octogenarians are grandparents. While one octogenarian might live in an assisted living residence and use a wheelchair, plenty of octogenarian folks drive their own cars and live in houses or apartments. Just like teenagers or thirty-somethings, octogenarians don't necessarily have much in common with each other aside from their age.