At 85 years of age, Roger married Jenny, a lovely 25 year old.

Since her new husband is so old, Jenny decides that after their wedding she and Roger should have separate bedrooms, because she is concerned that her new but aged husband may overexert himself if they spend the entire night together.
After the wedding festivities Jenny prepares herself for bed and the expected knock" on the door. Sure enough the knock comes, the door opens and there is Roger, her 85 year old groom, ready for action.
They unite as one.

All goes well, Roger takes leave of his bride, and she prepares to go to sleep. After a few minutes, Jenny hears another knock on her bedroom door, and it's Roger. Again he is ready for more "action".
Somewhat surprised, Jenny consents for more coupling. When the newly weds are done, Roger kisses his bride, bids her a fond good night and leaves. She is set to go to sleep again, but, aha you guessed it - Roger Is back again, rapping on the door, and is as fresh as a 25-year-old, ready for more "action".
And, once more they enjoy each other. But as Roger gets set to leave again, his young bride says to him, "I am thoroughly impressed that at your age you can perform so well and so often. I have been with guys less than a third of your age who were only good once. You are truly a great lover, Roger."
Roger, somewhat embarrassed, turns to Jenny and says: "You mean I was here already?"
The moral of the story: Don't be afraid of getting old, Alzheimer's has its advantages.