Girls' Ages

What is the difference between girls aged: 8, 18, 28, 38, 48, 58, 68, & 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 - You tell her 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?