story telling

maybe you've read this before but yet still funny though..
whats the difference between girls aged 8, 18, 28, 38, 48, 58 & 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 dont need to tell her a story to take her to bed.

At 38 - she tell 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 will be a story....