In Islamic legal terminology, bāligh (Arabic: بالغ, adult) or mukallāf (Arabic: مكلف, responsible) of muhallāq (Arabic: محلاق, tendril, mentally matured) or murāhiq (Arabic: مراهق, frequently errant, evildoer in a hurry) or muhtalim (Arabic: محتلم, pubescent) refers to someone who has reached maturity or puberty, and has full responsibility under Islamic law.
The previous answer is complete; I just wanted to add that "not being mentally ready" could mean a number of different things.
My first impression when reading that was that it could mean she is not mentally ready to wear the hijab in a minority society (or a Muslim area where hijab is uncommon) - for instance, at a school where there are no other girls who wear hijab, and she might be under pressure to explain herself (but not be able to do so yet), or be teased, ostracized, or bullied, especially if she is shy or sensitive and does not have an assertive personality or a strong ability to stand up for herself.
In some places, Islamophobic harassment might also be a concern.
Not that it necessarily has anything to do with her personal sexuality.
I just wanted to add that because sometimes there is a tendency to discuss the hijab wholly in the context of sexuality or modesty, wheres in minority societies, the main challenges and pressures regarding hijab are usually social and relate to things like Islamophobia.
In any case, it is good to acknowledge, respect, and nurture the inherent maturity of young people. Even if they are still maturing in many ways, throughout much of history, young people have taken on many lifelong commitments at a young age, such as apprenticing to a profession, training in sports or the arts, or a religious conversion. Of course it is also good to acknowledge the limits of a child's maturity, since one doesn't expect someone who is 9 to be mature in every way. Still, in this day and age, in some societies, everyone who is under 18 is treated as a child which does not benefit them either; it is good to have a balance.
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.
Baligh person is no more a child in Islam. Once the child reaches the Bolough, he/she is no more a child even if the society calls them children. Baligh person has the responsibility to perform every obligatory act or worship according to his abilities. Baligh young person is responsible to do Amr bil Ma'roof and Naahi 'Anil Munkar according to his knowledge and abilities and the possibility of effect.
Dealing with parents must be with full respect even if they do what you think it wrong. You must avoid any word or behaviour which can cause hurting their feelings. Hurting the feelings of your parents or any of them is a major sin.
No Kaffara and no responsibility on a child in the age of three or four. Estighfaar is always good.
A "step-father" should aim at trying to become a father figure for his wife's children, and treat them like his own children. One positive way of treating children is by showing affection and loving care, through kissing the children, hugging them, and treating them in the best way possible.
And Allah knows best.