Fathwa, - When must a Muslim girl wear the hijab?

Question:
When is it really permissible for a Muslim girl to wear the hijab?
Does it matter when she herself is ready to wear it or is there an age
where you would have to?
Answer:
In the Name of Allah, Most Gracious, Most Merciful
Praise be to Allah. May the peace and blessings of Allah shower upon
our Beloved Messenger, his family, companions, and those who follow
them.
Dear Sister,
It is permissible for a Muslim girl to wear hijab any time she
pleases, even if she is not old enough to have to wear it. As to when
she has to wear it, a Muslim female is required to observe hijab as
soon as she reaches the age of puberty, that is, when she sees her
first menstruation. However, it is highly desirable that she be
introduced to hijab before puberty. Sometimes, if a girl waits until
she reaches puberty, then puts on hijab all at once, she can find it
difficult. Hijab is best introduced in stages, say, first a scarf,
then the long sleeves, long skirts, etc. If it is done this way, along
with strengthening one's iman and love for Allah, then insha'Allah
when puberty comes, there should be no difficulty.
As to when you feel ready, readiness can vary with the individual.
Some people feel ready right away, and some take a lifetime, and some
never feel ready. So don't base your decision to wear hijab completely
on readiness. If you do, you may miss out on the immediate benefits of
wearing it.
Strengthen your iman, establish your prayers, and constantly remember
Allah. Make small but significant changes to your wardrobe, like
making your clothes longer and looser. When you cover for prayer, keep
your scarf on longer, instead of taking it off. These are steps that
can increase your himma or enthusiasm for hijab. And keep in mind that
the Prophet, peace be upon him, said in a Hadith Qudsi, "God Most High
says I am as my Servant thinks I am. I am with him when he makes
mention of Me. If he makes mention of Me by himself, I make mention of
him to Myself. If he makes mention of Me in an assembly, I make
mention of him in an assembly better than his. If he comes closer to
Me a hand span, I come closer to him arms-length, if he comes to Me
walking, I come to him running."
And Allah alone gives success. And Allah knows best.