The worldwide umbrella body the APA - American Polygraph Association has a model policy updated on 4 Sep 2021 which states that there is NO minimum age to do a test. Previously they had suggested over the age of 12. This is the policy wording:
Examinee suitability . Persons who are suitable to undergo polygraph testing should minimally
meet the following requirements:
4.1. Capacity to consent to the polygraph examination, as demonstrated by awareness and
understanding of the context or reason for the examination referral.
4.2. Possess a basic understanding of right from wrong, and the difference between truth and
lies, as demonstrated by an ability to verbalize potential reasons for being honest or
dishonest, and the potential rewards or consequences for dishonesty or truthfulness; and
4.3. Maintain consistent orientation to date, time, and location, for example: being able to
provide basic identifying information, (i.e., name, address, date, date of birth, etc.) to
complete an authorisation form.