Since about 6 months old my DD1 has been well over the 98th for everything. Now aged 11 she's 5ft6 and an adult size 16. That sounds fat, but she's not fat, she's just not a womans shape, she's a childs shape (waistless, chestless).
She's been wearing adult sizes since she was 8. This is the huge challenge when raising a tall daughter. You try finding pretty tops in an adult size which do not taper for the bust and waist she doesn't have, or which are not too low at the neck (the neck line comes up lower on her because she has no bust). I've just looked up tops/t-shirts on Nexts website and of the 24 displayed only two are possible on DD1. Similar search at M&S yeilds 3 of the first 24 for short sleved tops. New Look did a bit better, 4 of the first 20 t-shirts displayed were even a posibility. You get the idea.
They grow so fast too. They wont be in their clothes long. I am always amazed when I see my best friends 10 year old in the same clothes as she had on this time last year. This will never happen to a child of mine (we're a tall family). On the upside it means clothes rarely get chance to wear out, so selling them on offsets it.
Suitable adult size 9 black leather school shoes took me a week to sort out. I am a size 9.5 myself though so I had a head start. The High School Musical phase was awful for her, everyone had merchandised clothing but her. We couldn't even find her a t-shirt.
The other major challenge is how people perceive her. It was sad when I took her trick or treating at Halloween and one old cow opened the door and said "you must be joking, Halloween is for the littlies". "She's still in primary school!" I retorted. "Get on with ya" said the old cow. On the up side, she's been able to get in to see Harry Potter at the cinema with me since she was 8 too, even if it is meant to be a 12. (It's Ok, I watched it first to check.) That gave her much kudos at school for months, our cinema were really hard to con.
If there's ever a playground arguement she gets more severly in trouble for any bossy behaviour too, which is unfair, it's the same behaviour regardless of the size of the child doing it.