Period talk with my 8 year old

My step daughter will be 9 in august. She's already wearing bras and I'm contemplating if I should speak to her about the period and what to expect. There are already a few girls in her class that have started and I want her to be prepared for what's to come. The only thing that's holding me back is if she's old enough for me to break it down to her. I mean, she's only in the 3rd grade. So my question is, should I talk to her about it now or wait until she's a bit older.