What is the difference between girls aged: 8, 18, 28, 38, 48, 58, 68, and 78?

At 8 -- You take her to bed and tell her a story.
At 18 -- You tell her a story and take her to bed.
At 28 -- You don't need to tell her a story to take her to bed.
At 38 -- She tells you a story and takes you to bed.
At 48 -- She tells you a story to avoid going to bed.
At 58 -- You stay in bed to avoid her story.
At 68 -- If you take her to bed, that'll be a story!
At 78 -- What story??? What bed??? Who are you???