In this household we feel it depends on the child and the parents
Up to age 3 she can go for free, but if she still needs frequent naps, can’t take standing in line, or is afraid of strangers or large characters don’t take her until she is older. Ds was 7 the first time we went and for him it was a perfect age(large character issues until he was nearly five). He could ride any ride he wanted without us. He couldhave probably gone younger, but we waited. Like I said it depends on the child, but it also depends on how the parent (s) and how they deal with meltdowns–because they will happen.