Difference Between Girls

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

• 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!