Thank you for your question to planDisney! With theme park tickets (as well as water park tickets), an adult is someone 10 years old and older. A child is considered someone between the ages of 3 to 9. Children from 0 to 2 therefore can enter without a ticket.

At Walt Disney World and Disneyland, a "child" for the purposes of theme park admission is defined as someone aged 3 to 9. Children under the age of 3 are considered "guests of Mickey" and do not require a park ticket or a reservation; they can enter the parks for free. Once a guest reaches the age of 10, they are considered an "adult" in the eyes of Disney ticketing and must pay the full adult admission price. This age distinction also applies to most other Disney services: for Disney Dining Plans, the "child" price is for ages 3–9, while those 10 and older pay adult rates. Similarly, at buffet or "family-style" restaurants, the lower child pricing applies only to the 3–9 age bracket. It is worth noting that for some specific experiences, like the Bibbidi Bobbidi Boutique, the age range is slightly different (usually 3–12), but for the vast majority of your vacation planning—including park tickets, genie+, and meal plans—the jump from "child" to "adult" happens on the child's 10th birthday.