DD1 was born on 31/8, and has always been the youngest in her year. I don't think I even had the choice to hold her back a year either....
It took her a while to settle into school, and she is now in Yr2 - she's on the top table for literacy, with a reading age of 8yrs 10months (she's 6yrs, 7months), and she's attaining average results for literacy - as she is doing well in other areas, we are going to sign up to Kumon to bump up her literacy (but only if she enjoys it and wants to do it).
She has always been confident though, and I probably would worry more if I had a son, as I know that they can sometimes struggle more than girls.
Mind you, when she did first start school, I did worry about her progress - I did well at school, and couldn't get my head around my child not doing well. I had a bit of an epiphany, and realised as long as she was happy, and trying her hardest, that was all I could ask for.