Every school system is different. You can homeschool anytime you want in some states. I start at 6 months here in KY. I would say that you need to place your child in the grade that the child's education level is at and not the age level. Here in KY as far as public schooling the kids have to be 5 by October to enroll in K. but when you homeschool it is totally different. Do you know your state laws regarding homeschool and or your school districts laws? You need to assess your child to see what level he/she is really at and maybe not place a grade level on the child? If you need help assessing and knowing the guidelines of grades let me know. And if I may ask what curricula are you going to use to homeschool? However like I said you need to check your state's laws and districts laws about homeschooling and you can check with HSLDA (Home School Legal Defense Association for more info. and legal advice regarding homeschooling.
If you haven't picked a curricula out yet you may wanna consider
www.time4learning.com They assess your child and provide your state's standards.