@Sparklingbrook
How is it your point proved
*@SarahAndQuack*?
You seem to be comparing different things.
The ones that I am interested in will become adults in the next couple of years and Instagram will still be there. 
Yes, I am comparing different things, which illustrate that people's ideas about what's socially appropriate often change as technology changes.
My point is proved because you've never heard of Livejournal! It is (or was, I'm not sure) a pretty early internet forum, very popular with teens/young adults in its day, where people shared all sorts of personal info.
At the time we were all told this was very foolish and we'd all feel terrible shame in years to come, when we were responsible adults and all the livejournal users of the year 2021 would be able to access our silly teenage diaries at the click of a mouse.
Of course, what actually happened is that livejournal dropped out of use, just as instagram will, and although most of that content is still perfectly accessible, it might as well not exist for all the attention anyone pays to it.
I do see that there is a difference between a 13 year old posting for herself, and a 13 year old whose mum has put pictures of her up on social media, btw - they're not the same situations, but they're comparable for what they illustrate about humans and technology.