mankyscotslass - I was 7 and it was tough but note I am now 43 and apparently they have better ways of doing it now :)
I was in a hip-spiker cast from my armpits to toes with a metal bar bracing the knees. Apparently they can brace the knees and use a lot less plaster now? It WAS painful (they break your femurs) and I had a few days under a cage of lights to dry the plaster out - which was hot. Also, parents could only visit at set times, and I was so far away from home my Mum came every other day & my Dad once a week - so everything seemed a bit worse than it would now if your Mum is with you loads etc. I came home for a month with the cast on and had a trolley to scoot around the floor on. My Mum had to get the neighbour in a few times a day to help turn me (when need the loo etc!)
I had intensive Physio in the hospital and was allowed home when I could manage to walk & do a staircase with 2 walking sticks (went from daily swimming pool to get legs moving again, then walking between parallel bars, the crutches, then sticks).
I learned to ride my bike very quickly and was allowed to ride to school.
My feet & legs were straight and I could walk/run without permanently tripping. I avoided a wheelchair which was forecast without the op as my condition was progressive. I was useless at Sports days, but that is probably just me! 
I had a few further ops. At 16 my metal plates were removed and in my 20's I had a muscle released and bone shaved as my muscles had grown for so long in the wrong place, that as I grew and bones had been straightened I had a few muscle problems. My parents kept saying "maybe we shouldn't have put you through that" but I am eternally grateful they did!