Puberty is recognised by signs that have been defined by the scholars, one of which is the growth of hair in certain places on the body.
The hair which signals puberty is the pubic hair that grows around the penis in males and the vulva in females. With regard to armpit hair, the beard and moustache, the growth of any of these does not signal puberty. Moreover, it is not just any hair growing in the pubic region that signals puberty; rather what counts in this case is that the hair should be coarse and need to be shaved with a razor and the like. In the case of fine hair that does not need to be shaved, this does not signal puberty.
If any of the signs of puberty have appeared in your daughters, as mentioned above, then this means that whoever has these signs appear in her body has reached the age of accountability and responsibility.
There is no negative impact for you or your daughters, in sha Allah; rather this is something that Allah has decreed for them, and we have to cope with it by adhering to the limits set by Allah, not transgressing them, and by respecting them. What you have to do is raise your daughters in that manner, and pay attention to them, advise them, teach them and ask Allah to protect them from error and not to burden them with more than they can bear. You will be rewarded for all of that, in sha Allah.
However it is very possible that there is some kind of imbalance in their hormones or glands that has led to this, because the appearance of these signs at an early age gives rise to concern. Hence we advise you to take your daughters to a specialist doctor, because perhaps they need medical attention.
May Allah help you and protect your daughters, and cause them to grow up well and strong, and may He help you to give them a good Islamic upbringing.
And Allah knows best.
Reproduced from Islam QA