FFS people. The world is not the finely tuned binary a lot of you seem to believe...

[RC]

86. What no one here seems to be addressing is the fact that the age of adulthood used to be 21, period.

Then you could vote, drink, smoke, and screw around with anyone willing, who was also 21 or over, starting with your 21st birthday.
Then the Viet Nam war happened and the age of maturity was dropped to 18, three years younger, because that was the age you, as a male, could be drafted for canon fodder and you had no vote in the matter, because you were not yet 21. No other reason.
But, you were still not fully an adult, in the eyes of the law. Laws prohibiting smoking and booze until 21, were still in affect some places. Other places allowed one or the other, or both at 18.
The 'Who Could Fuck Who Laws' also had to be adjusted. After all, that immature 18 year old, teenager was suddenly a mature adult now, with the passage of the law applying "Adulthood" at 18.
No longer could an 18 year old safely date a 14 or 15 year old. Or sometimes even a 16 year old, even though you could still get married at those ages, depending on the state. Some states had the age of consent at 14, even with someone over the age of 21. It was all good, in whatever state, at the time, till some blue noses tried to set the age of consent, hard lined at 18 - And they took the booze and cigarettes away too.
It all eventually shook out to what we have now. It is still shaking.

What escapes many here is the fact relationships are not stamped out like cookies. We do not all 'mature' at the same rate, or even to the same levels of maturity, no matter how old we are, or may get to be. Each relationship is unique and changing. At best, they only loosely follow some formula or another. Age, maturity, personality, experiences of both, all play apart. What works with one couple, is detrimental to another. What is glue for some couples, is a solvent that ends the relationship for others.
Just because it doesn't work for you, doesn't mean it can't work for others. Just because you are 18 or 21, does not necessarily mean you are mature. Just because you are only 14 or 15 does not mean you are not mature. It depends on the individual. And sometimes the situation.