I allowed my mother to take my son when he was 4 yrs old,he doesn't remember much,but he remembers he couldn't ride what he wanted to because of restrictions (height),since he was with his Nanna and at that age he wasn't hanging out too late.
Now my son is 13 and it's a different story,we're hop all over to all the attractions and love it. We now take advantage of the Passes and hang out doing things til they close.
So I say younger if hanging with the grandparents,they will wear each other out and sleep very well.Alittle older if you want to hangout,ride,eat and see shows.
My son loves rollercoasters took him a couple of years but now it's on,so I had to get with the program.
We enjoy our adventures,it's really cool to enjoy fun things with your children.