To sign an Affidavit, a person must be of sound mind, which means they have the mental capability (or capacity) to understand the statement itself and the implications, such as perjury, of swearing the document.
Typically, Affidavits are signed by a person who is over the age of 18. However, there is no minimum age requirement, and in some instances a minor could be required to swear an Affidavit, for example, to submit evidence in a family law proceeding.