I've done some more research. Age of consent laws being pushed to age 16-18 seems to predate the porn industry, which wasn't legalized until the 1970s. Indeed, it was not illegal to have minors in pornographic productions until 1978 largely because porn itself wasn't legal until shortly before that, and a "minor" was considered anyone under the age of 16. This was not increased to age 18 until reforms made under Reagan in 1984.

Age of consent laws existed in some places in the 11th and 12th centuries, but they were much lower than 18, and generally coincided with the age of marriage, which in turn generally coincided with the age of puberty. It was hard to enforce anything on the basis of age back then, though, since age often wasn't tracked accurately and there weren't always birth records.

It wasn't until the last couple hundred years that the age of consent laws started creeping higher, and it was due to outrage against young girls being sold as prostitutes. According to Wikipedia, English common law generally set the age of consent to between 10-12, but an investigative journalist by the name of William Thomas Stead exposed what was happening with the underground prostitution of young girls and the "respectable middle-class" was so horrified by reading his accounts that they pushed for reforms, including increasing the age of consent to 16 in 1885. The US followed, with laws in various states raising the age of consent to 16-18 by 1920.