Fact from fiction, truth from diction. Getting to just the meat of the logic of your question –as you have asked devoid of much emotion, this is my best and logical guess. When a child is no longer a child BIOLOGICALY is when a person can become a father or mother. Biology set that clock around 14 to 16 with most humans on average. That does not sit well with humans because humans feel the mental capacities of these young adults are not mature or stable enough. That us usually the slippery slope because there is no unilateral standard on when or what this level of maturity is; are when the person leaves childhood ad enters adulthood. Here in the US it is more geared behind the buck, When dollars are at stake it benefits business and society to see a 12yr old as an adult you can charge them more money for tickets or movie passes etc and increase their profit.
Europe has far been more mature (overall) in the areas of sex than the US, which could be due to the nation’s roots in Judea Christian beliefs. Many acts of sex were illegal and punishable, remember the scarlet letters? I believe Europe and many other parts of the globe follow biology closer than Christian doctrine. That gives the illusion or maybe not so much that Yankee kids are behind even while they seek to be out boinking like bunnies. But there might be merit to it, if Europe treats its young adults as being capable to vote, drink, do a porno at younger ages than 18 they may rise up to the occasion. Such as if you never expected a person to learn above the 6th grade level would they?
The US it boils down to more emotion than physical logic. A 16 year old cannot make a porno, see one, much less act in one; to do so they would have to have sex with those above 18; surely she would not understand or know enough about sex and be played by these older men, better she be played by guys her own age. She can boink all she wants with another 16yr old boy and have it be de factoly legal. No doubt they will give her adult props when she goes to the amusement park or need a train ticket.
The slippery slope is that maturity, and or being able to have the intelligence to consent to sex is not constant. For the most part it is believed when you hit the magic age of 18 you have enough intelligence to determine who and how to have sex (with anybody not just others who my be just as ignorant as you on the subject). However that 18 year benchmark is tossed out IF the IQ level of the person doesn’t keep pace with the body, if a person has a birth defect that causes them never to be able to obtain an IQ lever above that of a 10yr old they can be here on this planet 37 years and longer and never be seen as sentient enough to direct there own course. The blanket 18yr maturity benchmark is dashed to pieces; it is feckless and wishy washy, because it is not being used as an absolute but a sliding scale which makes it manipulability quite easy. Conclusion, the US slides the scale more by emotion then biology because they can stomach it better when government, society and business can manipulate when a person is or not an adult to further an agenda.