Theme parks define an adult (for entrance to the park purposes), as anyone 10 years old and over, without age limit; a child, again for ticket purposes, is anyone between 3 to 9 years of age; infants under 3 years old, do not need a ticket to enter the park and enjoy the attractions (height restrictions on some rides may apply).

This has nothing to do with the minimum age required to check in at a hotel; some hotels define the minimum age to check in as 18 years old while the vast majority requires 21 years.

The age requirements are in force on the date you go to the parks (or check-in at the hotel), not when you make the reservation.