My daughter went when she was 4. There was a girl in line who was about 7ish and refused to go on it. Some kids are just traumatized by the sounds of the fast cars and don't realize..it is similar to riding in their own car (if it were a convertible!).
I think once kids g on it, they woudl be fine. But there was no amount of convincing by my small fry that would get that kid to ride it and when we got to the car the girl started crying and had to be taken out.
I don't think the ride is intense--I think the intensity is an illusion--but if you cannot get your kid safely into the car without them getting upset or hysterical--then the answer is...the ride, for now..is too intense for them.
ETA: For clarification--my small fry LOVES Test track!