I think it very much depends on your child.
DS has a mid-July b'day, he didn't start in SEptember after he was 4 as our schools have a twice-yearly intake, but when he started in the January before he was 5 he was one of the youngest in the class, but also more than ready to start school.
He suddenly grew up in the last 3 months before he started school - when he got his place, I was sure he was 'too young'. By the time he started, he was very ready. Because of the January intake, most of DS's freinds are the young boys in the calss, too - and they are all doing very well.
In an English school, Reception is v gentle, learning through play etc. I think he would definitely have outgrown the pre-school nursery he was at and found it boring or 'babyish' - especially if he was with much younger children.
DS is doing well. He has been slower to read than I expected he would, but I have just shrugged and thought 'well, he's very young in his year'. And he's doing very well now, once it clicked.
If your child is unsure of himself, very shy or nervous and / or you think he might struggle academically, then it might be best to hold him back - all depends on the child.