The ideas which inspired the Declaration of Independence were thought to apply to all people. It goes back to the ideas of the social contract and the purpose of the government. That's why the Americans felt justified in breaking from Britain in what some consider the first revolution of their kind in history... men such as Jefferson really believed that the government was not protecting the rights they believed all men had. Maybe they were justified or maybe they weren't, but this was the justification they used to break from Britain. Others have used these same ideas to break away from tyrannical rule far worse or overthrow previous governments.
"Life, Liberty and the pursuit of Happiness." is such a wooly useless statement though.
You could argue that based on the above, the US shouldn't have a minimum age limit for alcohol since it clearly infringes on teenagers (heck even college kids) pursuit of happiness.
I'm not arguing that it's correct that they have strict laws about pornography, nor would I argue that it's incorrect that the US has strict laws on alcohol. It's up to each country where to set its own laws.
I'm not being sarcastic. I'm a libertarian (i.e. classical liberal). I think people should have freedom to do things that don't hurt others.
It's only legitimate to stop people drinking when there's some danger involved, e.g. drinking with driving. One can argue for other cases like drinking heavily with a "mean drunk" personality type, or drinking heavily without the experience/knowledge to be safe in that situation. Drinking with being aged 20 isn't inherently dangerous. And it's a bad (both inaccurate and ageist) proxy for the other things that actually matter.
Mind. Blown.