How old am I

 Three mischievous grandmas were sitting on a bench
 outside a nursing home.
 About then a grandpa walked by, and one of
 the old grandmas yelled out saying, "We bet we can tell exactly how old
you are."
 
The old man said, "There ain't no way you can
guess it, you old fools."
 
One of the ornery grandmas said, "Sure we
can!  Just drop your under shorts and we can tell your exact age."
Embarrassed just a little, he dropped his drawers. The grandmas
stared at him for a while, asked him to turn around
a couple of times, asked him to jump up and down for a little while
and then they all piped up and said, "You're 84 years old!"
 
"How in the world did you guess?!?"
 
The ornery old grandmas, snickered and
 laughed.  Slapping their knees and grinning from ear to ear, all three
happily yelled in unison, "Because we were at your birthday party 
yesterday.