Simple age restriction guard that only relies on js-cookie.

js-only-age-restriction.js

// Make sure js-cookies are loaded before this code
function verifyAge(gaurdingPage = "./age-verification.html") {
	if (!Cookies.get("age-verification"))
	{
		Cookies.set("intended", window.location.href);
		window.location.replace(gaurdingPage);
	}
}

function checkAge(homePage = "./index.html", awayPage = "https://www.google.com/?q=") {
	function goToIntendedOrDefault(defaultAddress = "./index.html") {
		var current_intent = Cookies.get("intended");
		if (!current_intent)
		{
			window.location.replace(defaultAddress);
		}
		else
		{
			Cookies.remove("intended");
			window.location.replace(current_intent);
		}
	}
	if (Cookies.get("age-verification"))
	{
		goToIntendedOrDefault(homePage);
	}
	document.getElementById("enter").addEventListener("click", function(event)
	{
		Cookies.set("age-verification", true, {expires: 30});
		goToIntendedOrDefault();
	});
	document.getElementById("leave").addEventListener("click", function(event)
	{
		Cookies.set("age-verification", false, {expires: 30});
		window.location.assign(awayPage)
	});
}

/**
Usage:
	In every page, simply use verifyAge with the guard page address.
	In the guard page, make buttons with id="enter" for verified and id="leave" for not verified and call checkAge with your home page address, and an address to redirect people when they are not verified.