Most folks hit puberty at around 13. Imagine your parents have divorced -- your new stepfather is very religious. Your phone and laptop have spyways ("parenting software") on them. You manage to get onto a terminal at the public library. You've missed your period -- you're afraid you're pregnant, and not sure how much time you have to do something about it.
There are so many edge cases where the benefits of access to social media outweigh the harms -- but we've framed this as a discussion about selfies and sharing when it's really about free expression, and there are so many dark turns a young life can take that are made darker if they're left to their family and friends to rely on for help.