Say no as long as you can...seriously. I do not like snapchat at all. My oldest is only 12 and hasn't asked for it yet (luckily). He had twitter for a school project (long story) and I had him delete it immediately after. We had 2 18-year-old boys living with us last school year for an exchange program. Trust me, snapchat is used for all the things you don't want kids to use it for. There's this false idea that the snaps are completely gone after you've viewed them, but anyone you send it to can screenshot it (and they do) and then post that pic wherever they want. Nothing disappears from the internet and until a kid can understand that, he/she is not old enough to use the app. Just google "snapchat nudes" and you'll see what I mean. Terrifying. I closely monitor my children's use of the internet (although I am by no means a helicopter mom - the internet just really frightens me).
eta: snapchat is extremely addictive. The teenagers that lived with us were constantly on it (and twitter). It was disturbing how much time they spent looking at their phones and really opened my eyes to social media use among teenagers.
Karen
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Mom to 2 hockey-playing, Lego-loving boys DS1 2003 & DS2 2005