Variables utilized in human decision making should be based in the natural world rather than arbitrary selection.
An example of this is consent. In order for consent to be actually given, the individual has to be fully cognizant of the situation and not coerced. Being cognizant would, theoretically, require a fully developed brain - although this can’t be accurately assessed on-demand and on a case-by-case basis, the age for consent (alcohol, military service, driving, sex, etc.) should be placed on the average (at the time of this writing I believe that occurs around 25 years old).