However I do believe that you've found the correct site to use as a reference.
If you follow the link through to the Arkansas page, it becomes easier to understand.
Some of the definitions on the site you mention are slightly misleading because the law is changing, but this mostly affects gay or lesbian couples.
The law regarding heterosexual couples is far more clear.
Brooks engaged in sexually explicit messages with Casper in which he said he hoped "that your coochie and her's are mine." Brooks responded by saying "Yes baby we are yours" along with a photo of the baby lying naked on the open diaper.
In many cases, a person between the ages of 14 and 16 can consent to sex with a person who is no more than 3 years older.
Those images showed a naked baby on an open diaper.
Some images included a sex toy on top of the baby's genitals.
Generally, when both parties are over 16 years of age, there will be no age-related legal issues concerning the validity of the consent between them.
Legal issues arise when a person is under the age of consent.