Lalalalove had a good answer. You really can't predict it that far in the future. You can guess from the size of your boobs, discharge, and when your mom got it though. I can't even predict an age from your description. My cousin got hers three years earlier than her mom, so that rule doesn't always work. The average age is 12-13, bit between 9 and 16 is normal. I got mine this year and I'm 12, but my cousin got hers at 10. I know that you don't want to hear that it's unpredictable, but it is.
This site really prepared me. It doesn't have pictures and it isn't inappropriate.
www.being girl.com