DD1 started when she was 4 turning 5 in June. We ummmed and ahhhhed about it for ages. Preschool said they thought she was ready but they were concerned about her shyness. That wasn't going to change in 1 year though. So, we enrolled her and asked the school teachers what they thought during Kindergarten Orientation and they thought she was ready.
The year she started she was the 2nd youngest in the whole year with the youngest being born 1 day before her. Some of her friends turned 5 in march and april though. But yeah many others were 1 year older. She is doing well. Started High school at 11yrs turning 12yrs and is in Year 8 now. There has been some "friends" issues which you can get unfortunately but even though DD1 isn't 13 yet she is still more emotionally mature than some. She did have some bullying issues in primary also which maybe she might have been able to handle better if she was older but who knows.
When DD2 & DD3 started they were November so I was relieved not to have to make that decision again. And the year they started there was actually heaps of children who were 4 turning 5.