This article actually took me by surprise, but it's a long time since I've been a teenager:
http://www.nytimes.com/2012/01/18/us...mid=tw-nytimes“It’s a sign of trust,” Tiffany Carandang, a high school senior in San Francisco, said of the decision she and her boyfriend made several months ago to share passwords for e-mail and Facebook. “I have nothing to hide from him, and he has nothing to hide from me.”
Years ago I had a sick friend who gave his brother his password and he cleaned out his checking account.
Yikes, don't do it!