I first noticed changes in dd1 at around 8.
dd2 is 8 now and no evidence at all. So it is young but not unheard of I think.
dd1 went through smelly armpits as her first sign of changes to come, then boobs started developing and then body hair.
She is now 11 and is quite well developed in some areas, definite small breasts, armpit hair and pubic hair. Her head head has gone greasy over the last year and we have had to change to a more adult shampoo, and she often stinks even when she has had a shower and used deodorant.
She is tall for her age and not skinny, both of which have played a role.
But despite all that, she hasn't started her periods yet.
She has had soft crop tops for ages, rather than a bra, and even now, at 11 with definite breast shapes, she doesn't really need a bra, but she now has a couple of simple ones, because she wanted to wear one. We found Asda crop tops were the best, soft, sporty shape, so wider straps and racer back, not silly spaghetti strings that slip off the shoulder, and very comfortable.
When I noticed the first signs I bought her a couple of books about puberty and body changes. There are some nice ones out there. I let her have them to dip in and out as she wanted to.
They say about 2 years from pubic hair to periods starting is typical, but not always, and some kids don't develop in the same order.
As for being a good mum, talk about it, treat it as normal, be matter of fact and laid back, give her good information (books) that she can read on her own and ask you questions, and then the door is open and she can come to you.