Hi, The typical starting age for kindergarten in the U.S. is age 5. However, you should consult with the department of education for your state to confirm the age requirement for your child. I would also contact the local school district to find out their cut-off date for enrollment (some districts require child to be age 5 by a certain date within the year of enrollment).
You can do a search on the web to find the contact information and/or website for your state's department of education (e.g., search "California Department of Education" on Google). SchoolFinder is an Education.com resource you can use to find the contact info for local school districts: http://www.education.com/schoolfinder/
I'm including below some additional resources you may also find helpful for this stage of your child's education and development. I hope all goes well with the transition into kindergarten.