Would it be illegal to distribute sexual photographs of myself as a child for free or in exchange for money given that I am an adult now?
The US child porn law is Chapter 110 of Title 18. It refers to a "minor", defined as anyone under 18. Sexually explicit conduct is defined, which you can read about. Regarding distribution (not production), 18 USC 2252 (A)(1) identifies as an offender anyone who
knowingly transports or ships using any means or facility of interstate or foreign commerce or in or affecting interstate or foreign commerce by any means including by computer or mails, any visual depiction, if—
(A) the producing of such visual depiction involves the use of a minor engaging in sexually explicit conduct; and
(B) such visual depiction is of such conduct;
"Affecting interstate commerce" is a term of legal art that means "doing, at all", which relates the law to the Commerce Clause and givers the federal government jurisdiction. There is a big interstate trade in child porn, and absolutely restricting distribution of child porn is part of what's necessary to regulate this form of interstate commerce (see Wickard v. Filburn, 317 U.S. 111 (1942): "affecting interstate commerce" can be found even when a farmer grown wheat for home consumption).
There is no overriding of the law in case the minor is now an adult, and there is no overriding of the law if the producer, purchaser or distributor is the child in the porno.
The core prohibition is 18 USC 2251, and distribution is covered in (d)(1), which defines the person to be punished as one who
knowingly makes, prints, or publishes, or causes to be made, printed, or published, any notice or advertisement seeking or offering—
(A) to receive, exchange, buy, produce, display, distribute, or reproduce, any visual depiction, if the production of such visual depiction involves the use of a minor engaging in sexually explicit conduct and such visual depiction is of such conduct; or
(B) participation in any act of sexually explicit conduct by or with any minor for the purpose of producing a visual depiction of such conduct;
In New South Wales, Australia this would make you a sex offender and, if convicted, put you on the sex offender's register for life. It doesn't matter if the child pornography is of you and it doesn't matter that you are now an adult and can give consent - the fact is that it is child pornography and the distribution of that is a crime under the Crimes Act 1900.