Should I be mad I'm a year behind?

No you shouldn't be mad. Everything happens for a reason and you cant change the past. Your age isn't uncommon for your grade. I was 18 during my senior and so was the majority of the senior class. You'll be able to tell all your friends that you're legal and they are not. It doesn't sound like you're that far behind. Is your birthday late? That may have something to do with it. In general, getting upset over a situation as silly as this is plain foolish. Most people are 18 when they graduate hs.