Edit: I was sort of right, but I found this on the website:
“We chose fourth graders because research shows that kids ages 9 to 11 are beginning to learn about the world around them. They're open to new ideas, and they are likely to connect to nature and our history,”
Original answer:
5th grade is the year that students learn about US History. I think the theory is that they get a chance to see the national parks before they learn about them in 5th grade.
There are also free passes specifically for 4th graders (and their families) for some reason: https://everykidoutdoors.gov/index.htm