I agree that many private schools aren't rigid in their year groups. A friend of mine's birthday was 1 September, but as she went to the High School from Kindergarten upwards, she was allowed to move up into the higher year.

My birthday is July, so I was always one of the youngest. At our state comprehensive, I managed to skip a year by doing an accelerated O level course and going straight from 4th year to 6th form, so was only 6 weeks older than some kids doing O level when I took my A levels. The disadvantage of doing that rather than moving up earlier was that we missed a whole year of the Hard Sums and practicals that were the bedrock of A level Maths & Fizzicks.