Anyone can purchase comic books, regardless of their age. However, the content of some comic books may be more appropriate for older readers, and some retailers may have their own policies regarding the sale of certain comics to minors.
For example, a comic book store might have a policy of not selling comics rated "Mature Readers" to anyone under the age of 18.
Ultimately, it is up to the individual retailer and the parent or guardian to decide which comics are appropriate for a particular child.