I have a daughter who is under one... but in general, Muslim girls cover when they are praying and then once they reach puberty. If they go to a Muslim school, some have them wear hijab as part of their uniform... and others keep it for only the older girls.
Among my friends, most of their daughters started wearing hijab around 10 or 11 to imitate their Moms or big Sisters... some never wore hijab... and still others decided to wear it when they were older (15, 16, 17).
I have two boys (4 and 2) and they play with my hijabs... so I expect my daughter will to. Think of them is always available play silks.
Oh... some girls will start wearing it when they are eleven or twelve at the beginning of the school year, because they don't want everybody to know when they got their first period. (Girl isn't wearing hijab one day... shows up the next day in hijab.
) That's one of the reasons that some Muslim schools just make it mandatory for girls in 5th or 6th grade.