Hmm. You’re kind of wrong. I don’t know your grandparents, so maybe they aren’t having sex - but they would be the exception, not the norm.
Frequency of sex does drop after 60, but it doesn’t usually stop. Typically, a couple over 60 will have sex at least a couple of times a month.
Sex definitely changes. It’s less goal oriented and more about the journey. It’s a slow burn instead of fiery moments. It’s less athletic, more focused on mutual pleasure. Grandma might need to kneel on a pillow to blow grandpa, but she can still send him to the moon. Actually, grandma and grandpa are probably seeking comfortable positions - bad knees, bad backs etc, but the sex can be amazing. Different than 40 years ago, but every bit as good - in some ways better.
May you live long enough to discover the sweetness of senior sex!