DD is 11. She has longish, fine, straight blonde hair and it is a fucking NIGHTMARE. It gets massive knots and tangles underneath the 'main' outwardly visible hair and that's the only bit I can get her to brush for herself (sensory issues AND dyspraxia, I sometimes have despairing visions of having to still do her hair for her when she's in 6th form).
We go through a cycle. I spend a good hour or more combing conditioner through, brushing out all the knots one by one with a tangle tamer AND a tangle teazer (each one has different merits), punctuated by shrieks, tears, screams of rage, flouncing off, coming back, telling me how much she hates me, why am I hurting her so much, what feels like half her hair coming away in the tangle brush, me threatening to shave it all off, saying this CANNOT get to this stage again, until finally we rinse through and she has a lovely, straight, soft shiny head of hair again. I plait it up and by the next morning it's still good, albeit looking like it's been crimped when it's let loose. I brush it through. Still good. She goes to school with it in a pony tail. She comes home and takes it out of the pony tail. By bedtime a few small knots have begun to form. I ask her to brush her hair. "Yeah, yeah." I ask her if she wants it plaited up. "No, it'll be fine." Given that she's 11, I don't really want to stand over her while she brushes, so I take her at her word. In the morning, the small knots are bigger. She disappears while I'm sorting out her packed lunch and insists that she's brushed her hair and put it in a pony tail. If we're in a rush (usually because she's mislaid her homework/gym kit/vital piece of equipment/parts of school uniform), I don't always have time to check it. On the surface, it will look fine. By the time she gets home and removes the hair elastic, her hair is indescribably knotted beneath the surface. Bathtime and the cycle begins again.
I think the longest it has been knot free, since she decided 2 years ago that she wanted to brush her own hair because I hurt her too much, has been 4 days.
Plaiting when wet does help, as it seems to keep its shape better. She hates it though, because it makes her hair look crimped. Sigh.