Why is the US so uptight to fix the age of consent at 18 whereas in the UK it is 16?

Try the angle of physiology and what do 16 year olds know about politics? They are sheep for the Left in many cases. The marches last summer proved they were easily manipulated by the Left. Why change it? What's to gain? They can work at campaign centers for school like I was obligated to do. I hated politics at that age. Now they have the Left telling them that the world will end. That's just terribly naive.

The US does not have an age of consent law, That is left up to the states. Some states are actually 15 and 16 too for legal consent.

This is a country that was once so uptight it tried banning alcoholic beverages, something people had been imbibing for 5000 years. 

But the age of consent is not because of that.  Some people are quite mature about their sexuality at 16. Others, like Trump, never mature no matter how old.  18 is when most are capable of making decisions about sex with some sort of rationality. At 16 the hormones are still raging and control much of what a person says or does.

In many states in the US the age of consent is 16

I live in a state where the age of consent is 16. Age of consent varies state to state in the US.

Most 16 year olds have still not developed enough emotionally and mentally to understand about sex. 

That's not true.

Each state determines its own age of consent, and that ranges from 16 to 18.

However, it is a hard 18 to be able to sign contracts.

Well at least we still have the freedom to hate.