I think it depends on the kid. I took my daughter, who is now 10, up around the snakes when she was 3-4. Nothing too serious never even put it in low but she got so freaked out looking out the window and not seeing the ground I had to call the wife to calm her down.
I took my son who is 7 now, on the snakes when he was 4 and he loved it, actually fell asleep on the way out. He has not missed a trip since then. I usually load up the night before we go on a trip, he'll wake me up at 5 in the morning wanting to go or sleep in the living room so I don't forget to take him
My daughter is starting to come around but you can still see the fear in her eyes on obstacles, she usually gets out with Mom.
Good luck hopefully you find out you have a new co pilot, I love mine and can't imagine going wheeling without him, I've even taught him to talk smack on my buddies Toyota