Squarespace Age Gate. Add this code globally: Settings > Advanced > Code Injection. Your "Age Gate" page needs the slug of "age-gate", or you need to edit that in the code (2 places).

squarespace-age-gate-global.js

<script>
var cookies;

/**
 * Returns the value of the cookie name.
 * @param {string} name
 * @returns {string}
 */
function readCookie(name,c,C,i){
	if (cookies && cookies[name]) { return cookies[name]; }

	c = document.cookie.split('; ');
	cookies = {};

	for (i = c.length-1; i >= 0; i--) {
	   C = c[i].split('=');
	   cookies[C[0]] = C[1];
	}
	return cookies[name];
}

/**
 * Checks if the cookie
 * @param {string} url
 * @returns {string}
 */
function checkAge(url) {
	var laChCookie = readCookie('legal');
	if (!laChCookie) {
		window.location.href = '/age-gate?redirect_to=' + encodeURIComponent(url);
	}
	return laChCookie;
}

document.addEventListener('DOMContentLoaded', function() {
	if (
		-1 === window.location.pathname.indexOf('age-gate')
		&& -1 === window.location.pathname.indexOf('config')
		&& (!window.frameElement || 'sqs-site-frame' !== window.frameElement.id)
	) {
		checkAge(document.URL);
	}
});
</script>