Sorry, I haven't read all the replies but thought I'd throw my 2cents in.

I started DD at school when she was 4yrs 10mths & she was ok at first but by grade 2 she was behind academically & socially she was getting more stressed. She struggled all through school & I requested at the end of grade 4 that she be kept back to give her a chance to catch up but the school denied my request. They felt it would harm her confidence. Mind you she was already experiencing anxiety due to bullying & struggling with her work.
This time last year we moved towns & DD was in grade 6 but no way ready for high school so I asked her new school if they could put her in grade 5 for the last term & let her do grade 6 again this year. They agreed & the difference has been amazing. She'll never be a rocket scientist but she's happy, confident & getting her work completed on time & with enjoyment. I can now say she's ready for high school.