As far as the minor goes, there really is nothing illegal about him purchasing the pornography as he, in the eyes of the law, is incapable of understanding the consequences of his actions. However, the lady that sold it to him is committing a crime. Using Texas as an example, the law reads as follows:
Sec. 43.24. SALE, DISTRIBUTION, OR DISPLAY OF HARMFUL MATERIAL TO MINOR. (a) For purposes of this section:
(1) "Minor" means an individual younger than 18 years.
(2) "Harmful material" means material whose dominant theme taken as a whole:
(A) appeals to the prurient interest of a minor, in sex, nudity, or excretion;
(B) is patently offensive to prevailing standards in the adult community as a whole with respect to what is suitable for minors; and
(C) is utterly without redeeming social value for minors.
(b) A person commits an offense if, knowing that the material is harmful:
(1) and knowing the person is a minor, he sells, distributes, exhibits, or possesses for sale, distribution, or exhibition to a minor harmful material;
(2) he displays harmful material and is reckless about whether a minor is present who will be offended or alarmed by the display; or
(3) he hires, employs, or uses a minor to do or accomplish or assist in doing or accomplishing any of the acts prohibited in Subsection (b)(1) or (b)(2).
(c) It is a defense to prosecution under this section that:
(1) the sale, distribution, or exhibition was by a person having scientific, educational, governmental, or other similar justification; or
(2) the sale, distribution, or exhibition was to a minor who was accompanied by a consenting parent, guardian, or spouse.
(d) An offense under this section is a Class A misdemeanor unless it is committed under Subsection (b)(3) in which event it is a felony of the third degree.