In most states you are required to be 18 in order to get your naval pierced without parental consent. However, with the consent of a parent you may be able to get it done much younger.
This depends on the willingness of the piercer, the approval and usually presence of your legal guardian, and any local laws. Usually for people under the age of eighteen a permission note and their presence is required.
You have to be 18. If you have your parents permission you can get it at 12 years old in some places. Your parent must be present and sign some stuff though.
This varies by state and parlor. Sometimes, you have to be eighteen and sometimes you can be sixteen with parental permission. Call the shop where you want to have it done; they can tell you what their age requirements are.