Here in Ontario, Canada we start earlier. My older DD was 4, but some kids are 3.5 when they start (as long as they turn 4 by the end of the year Dec 31).
This is for JK (Junior Kindergarten). Based on the responses in this thread of children being 5 or 6, I'm assuming that the posters are discussing SK (Senior Kindergarten).
Here in Ontario, you must be 4 by Dec. 31 for JK and 5 by Dec. 31 for SK. This would not work here, as by age 6, school is manadatory, so children are automatically placed in grade 1, as kindergarten is optional in our province.
As a teacher, I support student readiness, but I can't imagine children of age 5 or 6, not being in a kindergarten program. If one uses emotional maturity as the sole determination, then pretty much every child I know would not have been ready for kindergarten. That is what kindergarten programs are for, and that is why in our province, our government is moving to full day, every day paid for kindergarten programs for all children by the year 2013. We have many studies and stats that show us that kids who are in kindergarten programs show better school success, lower drop out rates, etc.
I also wanted to add that in my province, parents don't get to determine placement of children - it is solely based on age. It is not based on cognitive levels, maturity, etc. I'm a Spec Ed teacher, and very rarely, if ever (my board officially does not do it), hold kids back, nor skip kids, ever. Children are placed in age appropriate grades. There are pros and cons to each way, but having parents determine placement is way more problematic for us, so we don't do it. By age 6, all children must be in school, by law, and they are placed in grade 1, even if they have not attended kindergarten before.
My kids were in Montessori, so they started Casa level, which is a combination of JK and SK at 2.5 yrs for DD and 3 yrs. for DS.
Tiger