Which number is missing?

"Sixty is the worst age to be," said the 60-year-old.
"You always feel like you have to pee. And most of the time, you stand at the toilet and nothing comes out!"
"Ah, that's nothin'," said the 70-year-old. "When you're seventy, you can't even crap anymore. You take laxatives, then you sit on the toilet all day and nothin' comes out!"
"Actually," said the 80-year-old, "80 is the worst age of all!"
"Do you have trouble peeing too?" asked the 60-year-old.
"No, not really. I pee every morning at 6:00. I pee like a racehorse on a flat rock; no problem at all."
"Do you have trouble crapping?" asked the 70-year-old.
"No, I crap every morning at 6:30."
With great exasperation, the 60-year-old said, "Let me get this straight. You pee every morning at 6:00 and crap every morning at 6:30.
So what's so tough about being 80?"

"I don't wake up until 7:00!"