For example, the San Diego Comic-Con International, one of the largest Comic Cons in the world, does not have any age restrictions. However, children under the age of 12 must be accompanied by an adult.
Other Comic Cons may have different age restrictions. For example, the New York Comic Con requires children under the age of 13 to be accompanied by an adult.
It is always best to check the specific Comic Con event website for information on age requirements and any other restrictions.