UnderIce, my point was that there's nothing wrong with having things that are 11 year old on an 11 year old.
I know how bones are formed, and how hair grows- just that I don't find it disgusting that this girl has hair that is as old as she is.
I view it as part of her, same as her bones are, and see no reason to cut it, especially punatively, because she "should have a more practical style".
It's her hair, as much a part of her and her identity as her bones, or her nails, or her eyes, so why this awful attitude to trim it down and get rid of it.
It's hers, she wants to keep it. She does need to learn how to plait it, and sleep in plaits.
Maybe have a look at some websites OP to teach your DD how to manage her hair- you give her other advice I presume, on diet, tooth brushing, and getting enough sleep- hair care is no different.
My dd with long hair plaits her hair and sleeps in the plaits. She brushes it out at night, in bed, using her tangle tamer: she uses a spray of dry oil on the ends, and she wears the plaits all night. No tangles in the morning.
Rather than just suggesting chopping it all off, and donating it to some no doubt worthy charity(to assuage the guilt trip no doubt) why not accept her as she is, and leave her with her hair as she wants it, until she decides she wants to change it.
We don't all have to have shoulder length hair on our kids, with shorter hair for boys (ready for warfare), like some sort of cookie cut children.
Long hair doesn't have to be painful, just google some Sikh sites, or similar, and see how she can care for it and style it.
Maybe we should accept that this girl wants to keep her hair as it is, and suggest practical solutions as to how the OP could advise her dd on her hair care, rather than suggesting she cuts it- she doesn't want to cut it, so that's not really a helpful suggestion, is it?!
Embrace the difference!