Gifted Issues Discussion homepage Forums Learning Environments How compulsory is K?

I was just curious - is the Kindergarten year in the USA compulsory?

Until recently, here in South africa, a Grade 0 or K year was optional and many kids only started school at grade 1. (although in the private sector many many schools start with grade 000 (age 3 turning 4)

Now this Grade 0 year has just been made compulsory. So I was wondering if it's compulsory there in the US (and in other countries too)