Bedtime Stories

What differentiates females aged 8, 18, 28, 38, 48, 58, 68 and 78?

At 8 -- You put her to bed and read 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 the hell are you?