What is the age of grade 3 children in the US?

Depending on the child's date of birth, and if s/he started Kindergarten at age 5, s/he would be 8 or 9 years old when in grade 3. The trend in recent years for some people has been to put off Kindergarten until the child is 6 years old, so s/he would be 9 or 10 years old when in grade 3.