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. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.