My grandmother had hers done at 98 years old, now at about 105 she still loves her "new" knees.
That's wonderful! Tell your grandmother, "You go, girl!"
My surgeon complains that I am too young for knee replacement at not quite 60 but he admitted that I'm bone on bone and if the pain is more than I can handle it's time. The biggest worry is that they only last 15-20 years at best, so I'll have to get replacements in the future, more than once if I live as long as my Mom who turned 91 this spring. I just keep wearing joints out - both shoulders have had work, too.