Children start kindergarten at 5, sometimes 4-years-old (and turn five during the school year) but it depends on your state/school's policy. There's often a cutoff date where birthdays that fall after it those children have to start kindergarten the next year. There are also pre-kindergarten programs at some schools, and of course preschools.
If you know the school your child will go to or that you want your child to go to I suggest you contact them or go visit and find out what their specific info is. The schools around you should help if you have questions.
Our School Finder tool can help you get started too.
As already provided in the answer above, it will depend on the child's birthday and the school's cut off date. Most children will be 5 years old, while others may start at 4 and have a birthday before the cut off date, others may be 6 or ready to turn 6 depending on where their birthday falls and of course, children develop at different stages and just because a child is 5 years old, does not mean that they are ready to begin.
Contact the school that is in your district to find out important dates such as the Kindergarten screening date and registration dates. They will be able to see if your child's birthday falls within the cut-off dates of the school year and the screening will be able to identify if your child is indeed ready to begin.
Use the school finder link provided above, if you need to locate your attending school.
If in the US, it depends on what state you live in and your state's cutoff dates for beginning Kindergarten. Typically kids start at age 5, but sometimes (depending on birthdays, parental choices, etc) it could be 4 or 6.