It's important to tell our teenagers that we too felt that everyone else was allowed to do all the things that we weren't when we were their age.
My mother told me that part of being a teenager is boasting to your friends that you are allowed to wear make-up, stay out late, have a boyfriend or whatever else it is that is cool right now, but that most of them were just telling tall stories.
Most teenagers, whoever beautiful / handsome they are go through phases of feeling ugly, or at the very least that everyone only sees their big nose, flat bosom or whatever feature they are focussing on at the moment. I spent my teenage years convinced that my lower lip was too broad (it isn't) and DH spent his youth convinced everyone noticed his ears (they didn't.)
I am afraid that instagram, snap chat, facebook intensify these feelings as it is so easy to post pictures and everyone wants to be glamourous and have film star celebrity lives these days, if they are teenagers that is.
Our job is to make sure they know we love them, as they are, but discussing what suits a teenage girl seriously with her won't do any harm either, nor will telling her what features we were sad about at her age. Dig out some photos of yourself at your grandchildren's ages and see their reactions. They might well conclude that they are better off then we were!