Assuming you child beings kindergarten at age 4, 5 or 6, first grade would be 5, 6 or 7, second grade would be 6, 7 or 8, and third grade would be 7, 8 or 9 with most children spending the majority of that grade while being the middle age (5 for kindergarten, 6 for first grade...etc.)
A child has to be 5 years old by September 1 first to start kindergarten. If not they have to wait until the next year to start. So a first grader is 6 to 7, Second 7 to 8, third 8 to 9. My child is 8 going to third this next school year.
My daughter had to be 5 by September 1st, luckily the day after her birthday, so she could go in at the early age. Not the same with my son, though. He was the day after, so he had to wait a whole year. Ages are 5 to 6 for kindergarten, 6 to 7 for 1st grade, 7 to 8 for 2nd grade, and 8 to 9 for 3rd grade.
The exact age of first second and third grade are very easy. Most parents decide the ages are, 6 to 7 for grade 1, 7 to 8 for grade 2, and last and most grown up, 8 to 9 for grade 2. Please give a thumbs up and I hope this helped you a lot!