Inner and outer modesty, chastity and hijab is something Almighty God has mandated for all men and women.
It is necessary for us as Muslims to familiarise ourselves with the many benefits of inner and outer hijab, and the earlier we learn this the better.
Young boys need to be modest, and so do young girls, even though they might not be intellectually mature enough. As long as they are aware of their surroundings, and have entered into the age of religious maturity, they become responsible and accountable for what they do.
This is what bulugh and takleef means.
Furthermore, being modest and having hijab does not necessarily mean one is sexually active. It is a process of engaging with the natural and mental development of a male/female.
An Islamic lifestyle prepares us for the world, and whether this particular girl might be mentally ready or not, she must adapt to what it is that our religion wants from us, as the Almighty has accommodated to all that is required for us in our natural needs as humans.
A young girl who reaches the age of taklif will realise how beneficial and positive hijab is when she prioritises her spiritual and moral conduct over everything else, and she will realise how intellectually mature she becomes.
As for the age of bulugh, for a female it is completion of 9 lunar years. For a male, it is the occuring of one of the following, whichever one occurs first is the sign of his bulugh:
1. having a wet-dream.
2. growth of thick pubic hair.
3. completing of 15 lunar years.
And Allah knows best.