We did early K for DS at Kindercare (cheaper than most private K.) He started K there midyear when he turned 4. When we enrolled him in public school the next year, they started him in K and then he was moved to 1st midyear (after he turned 5.) There is a state law here that 1st graders must be at least 5 years old.

He's in 3rd grade now, and doing well.