The idea of teens sharing passwords didn't come out of thin air. In fact, it was normalized by adults. And not just any adult. This practice is the product of parental online safety norms.
In most households, it's quite common for young children to give their parents their passwords. With elementary and middle school youth, this is often a practical matter: children lose their passwords pretty quickly. Furthermore, most parents reasonably believe that young children should be supervised online. As tweens turn into teens, the narrative shifts. Some parents continue to require passwords be forked over, using explanations like "because I'm your mother." But many parents use the language of "trust" to explain why teens should share their passwords with them.