The average age of a fourth-grader in the United States is 9.
The age can change based on when the child was born and when they started school.
If a child goes through elementary school the way it is usually done, they are 9 years old when they start fourth grade. But things are changing in education right now, and more schools are starting to offer transitional kindergarten programmes for younger kids. Most TK programmes require students to be old enough to go to regular kindergarten, but the programmes are designed for younger students. If a child starts school in a TK programme, by the time he is in fourth grade, he will probably be 10 years old.