I agree that the "best" age is going to vary from family to family. For us, we prefer to go when the kids are not toddlers yet - so around 12 months or younger. That way, they go with the flow still. And then we don't like to return until the child is at leat 3.5. I did do a trip with a barely 3 year old and am so glad I didn't try it sooner with him! He was/is rather spirited though so that plays a part. When he went again at 3.5, it was a HUGE difference. (Our other child was 2 months for the first trip and 9 months for the second so she was pretty easy. Though 2 months means stopping to change a diaper several times a day - 9 months was easier).
We're going again in a few weeks with a 6.5, almost 4, and almost 1 year old. The plan is to not go back again for about 3 years so the youngest will be at that 3.5+ age range. (And so he'll be 40" easily based on his current growth pattern). For this trip, our almost 4 year old may or may not make the 40" rides - but I don't want to wait until baby is a toddler so this was the best time for us.