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.