You can get around age restriction on youtube by opening the link intended for embeding. Just exchange /watch?v= with /embed/ in the url. This only works if the creator of the video allowed embedding. This is the case for most videos. For Firefox I wrote a extension that does exactly this. You can install it here https://addons.mozilla.org/en-US/firefox/addon/ageless/ or take a look at the source code here https://github.com/2341/ageless.