One good indication is to think back to how old you were when your puberty began. It is not exact, but is an indication. Also, www.drgreen.com may have some information about it. Puberty begins lots earlier these days because of the better nutrition that we receive - or they receive. That is a general trend all over the world - in nations that have adequate food for the people.
Moodiness does go with the territory, both genetically and with puberty. My whole family tends toward being depressed - aunts and siblings - so we have learned to handle it. It came from both sides of our family - so we got a double dose. It hasn't hurt our lives, I think that it has made us more understanding of other people. ..and made us realize that we have a job to do with our own selves to read and learn about depression. So, it isn't really bad, it is the way that you handle it.
P.S. you can't just say - stop being moody either - be understanding and respectful of her feelings, after all she is a little girl and needs her mother now more than ever. Talk to her like you wanted to be talked with when you were a little girl.