We're going to Alaska this summer--my youngest DD will be 12. We started taking the kids on National Park trips when my youngest was 8. 8 was when I felt she would be safe enough for Yellowstone. We did the national parks in WY and SD. She talked about the Yellowstone trip for a couple of years, I'm pretty sure she still remembers that trip, especially when she sees pictures from it. We did the national parks of
UT and AZ when she was 10. I really feel that any child over 8 will remember some of the trip--especially if you take lots of pictures and look at them with the child over the years. I made Shutterfly photobooks from both those trips--so it's easy enough to look through at any time.
Enjoy!