When you get old, sex is just about irrelevant in a marriage.
Unless you're in a healthy marriage of course. Although you might not be aware of it, being intimate isn't about wiggling the little chap around for 3 seconds and then rolling over and going to sleep, but about a sensual bonding of two people deeply in love - and most certainly a lot of fun, whether you're actually married or not.
In healthy relationships, this will continue well into 'old age'.
Still, Snake the sex doctor, (you know why they call me Snake heh?), will help you out a little..
Take the old dear out for a nice romantic meal - buy her flowers, get those dudes with the violins to come play, order a $100 bottle of red, some oysters.. Take her home - run her a lavender and ylang ylang bath, give her a nice sensual back massage and then with all the grace and charisma you can muster, stick it to her.
Good luck.