What are the best ages to truly enjoy the parks?
Let's see - I'm 37.. so 37!
I am guessing you are asking about the under 18 crowd. Here's my take:
Under 5 years old - Witnessing their excitement, joy, astonishment makes all those emotions contagious. They may not remember the experience. Depending on your kids' temperament, the experience could be a little overwhelming.
5 - 8 years old is a fun age because the magic is still real. They can manage around the Parks a little easier and start to ride some of the "real rides"
8 - 11 years old remembers it, has an agenda and you'll likely have to turn over the map.
Over 12 years old really depends on your kids. Some kids still love it. Others find it a little boring since there aren't a lot of roller coasters.
Once I know more about your kids, I will recommend the best options.