The message that gets displayed when a user who is not logged in loads a video with age verification is the following: “This video or group may contain content that is inappropriate for some users, as flagged by YouTube’s user community. To view this video or group, please verify you are 18 or older by signing in or signing up.”
Numerous ways to bypass that age verification check have been posted which included using caches and proxies. The Firefox extension You Old Enough provides users with another way. The experimental add-on will check if the video on Youtube is flagged for age verification and open it immediately in a popup window.
This means that it is possible to watch all videos on Youtube that require age verification without having to login to the system.