Mikey was a kid, and wasn't interested in sex yet, but he knew that older people were, and thought something related to sex would be a fun prank.
He began to write down a wish, but said "I wish I know if that does what I want."
Hmm, he thought. He didn't think of it that way, but after finding out exactly how it worked, he decided to make the aging more natural, using the new person's age instead of the original person's age.
On the paper, he wrote "I wish that I would switch genitals with my opposite-sex parent, and that the new genitals worked perfectly and were the age of the new person but both of us grew as if we had our original genitals."
The first victim of Mikey's stickies was....