My daughter had the same thing happen, she is now 13, her period didnt start untill just started late last year, but they are not regular, now she hasnt had one in 3 or 4 months, she is overweight she weighs (170)
I was so worried when i saw this when she was so young, she was diagnosed with precocious puberty also..they told me to try to keep her weight down as weight also has a factor on how your body thinks your age is higher and periods could start earlier if the child is obese, but we didnt have much luck loosing weight and the periods still held off untill she was 12
here is a link about precocious puberty http://www.mayoclinic.com/health/precocious-puberty/DS00883 There is some consideration that soy products have been a source of bone age development, genetics, chemicals ..So much information it would be good if you researched it ..there are many opinions .Good Luck