Thing is, we already applied precautionnary principles in a very organic fashion: hight end phones have always been expensive, and very few kids got access to phones able to access messages, email, then social networks.
Be it Blackberry or Vodafone/docomo phones it took years before regular teenage kids could buy them in significant numbers. Same for the iPhone, it's not like millions of teens got their hand instantaneously on a 3G the minute it went out.
By the time it became widspread enough, we already had devices in the wild for a decade or so (for reference imode launched in 1999).
We might want to revise how smartphones are regulated and/or social media policies, but IMHO we're far away from the simple "we don't know let's be cautious" phase, and there is a burden of proof on what exactly needs to be regulated to what expected effect.