>>939351937 (OP)
The 1970s.
It isn’t easy to perceive from today, fifty years later, but during this time, in North America and Europe, there was a concerted effort to make pornography mainstream, and to make pornography featuring children acceptable. Some folks were okay with this; most were not.
It was after this time, starting in the 80s, that a conservative backlash spread across the developed world. Setting an age of consent, or increasing it if it was very young, became one of the first and easiest political projects of this movement. Who really is going to oppose making it unlawful to have sex with persons under the age of 16 or 18?
Later, the United States also made it a Federal crime to produce, distribute, or possess pornography featuring persons under age 18. This is where today we get the idea that 18 is the age at which a person can consent to sex. Really it’s the age at which they can consent to appear in pornography, but the distinction is lost in Internet discourse.
