Why does my friend lie about her age to us?

I've been friends with my friend for around 7 years, shes also mutual friend with my sister also, which is how i became to be her friend, she claims shes 21 but shes really 20, we have known for a few years about her real age but when we once asked her about it she said they made a mistake on her license, which we know isnt true,Its also not the only thing she lies about, also her middle name to,which she says its the same as my sisters middle name. We wana ask her but we just feel bad for her side that she has left it for this long, were embarrased to ask on her behalf. Whats the best approach for this.