I am currently going through this with my Dd(6) ,she is been monitored by a consultant at the local hospital,so I have read up as much as I can on the subject.
I have informed her school (mainly due to the pmt symptoms she suffers,)and was told that 3 girls that went up to year 3 in September have already started their periods,and the Junior school has sanitary bins in all the girls toilets now as they've noticed the girls are maturing earlier.
Any signs of puberty under the age of 8 are best to be investigated,ie seeing your Gp,referrals to hospital etc,but signs of puberty starting ie breast development,body hair ,odour etc from 8 years up is now classed as the'norm'.
My Gp told me girls are maturing earlier and earlier,and after the age of 8 is nothing to worry about.
Puberty takes around 5 years to complete with menstruation being the final symptom,it usually starts with body odour,breast development,acne,body hair then finally period,let me just say pmt and hormonal moods can start right at the beginning(trust me a 6 year olod with pmt is a very odd thing to see,and very distressing sometimes too)
I would say see your Gp or school nurse if your concerned in the slightest or just to put your mind at rest.
I hope Ive helped to reassure you abit.