There are actual apps you can download that allow you to save a photo or video, not just the screenshot. I'm actually pretty baffled that people still don't know about them. Renders snapchat useless.
I am a 22 year old kid and I can't tell you how many times this has happened to me. It even happens with snap chats actually, because of the apps that allow you to save a snap. I am noticing however that there seems to be a strong correlation between household income and internet behavior. My wealthier friends all have pristine online images, while the poorer ones tend to share more.