From the day you turn 3 years old, a ticket/season card for children 3-12 years old is required. From the day you turn 13 years old, an adult ticket/season card for 13 years and older is required to enter the park. Children 0-2 years are free of charge. Seniors are considered adults.
If you are going to buy an annual card for the coming year, it may be good to keep in mind that if the child turns 3 years old or alternatively 13 years old during the year, a card for the correct type is needed after the person has turned either 3 or 13 years old. It is not possible to upgrade a card during the season.