There is no strict age limitation. However, the chaotic celebration, entry, traffic, and massive amount of people, as well as the fire used to launch lanterns, can be intimidating or even dangerous for children under 12 years old. This of course depends on each child's maturity.
Children under 8 years old enter free, but no separate seats and sky lanterns are provided. Children of 8 and above are charged the same as adults and receive the same provision. You can buy a ticket for your child under 8 if you would like her or him to enjoy the same provision.