1. Age 5 if Kindergarten is mandatory in your state, age 6 if it is not.
2. American: 1-5 or 6 is elementary school or grade school. (Generally elementary schools have Kindergarten available also. Some places even have Pre-1 between K and 1.) 6-7 and usually 8 and sometimes also even 9 is middle school or intermediate school or sometimes even still called junior high school. 10-12 is high school. That's public school--free, compulsory education. There are many private schools that all follow their own numbering systems, and many homeschooled kids who do their own thing. Post-high school education is usually referred to as "college" although it more often takes place at a university. There are also technical institutes, arts academies, etc., etc. This is all completely different from the English schools.
3. Many American children age 2-4 go to preschool. There is public preschool for disadvantaged children. Preschool is not mandatory.
4. American Kindergarten is a place to smell the school smells, hear the school sounds, get used to the school rules and routines. It's usually shorter hours per week than the rest of the elementary school. It may be only morning or only afternoon, or only a few full days each week instead of all 5 days. Kindergarten classes sometimes have fewer students per teacher than the older grades. Kindergarten children are often kept separate from the rest of the children in the school--different art and music and Physical Education set-ups, different playground times, etc.
5. Talk to any parent. Read many books and websites.
6. I have none.
