Those, who would like to download a video, have already watched it (in most of the cases).
If they were able to watch before, that means they are old enough to watch it ==> and to download as well
Gotta agree. Especially in the case of videos involving political or sociological events, violence can and does occur, and such videos may indeed be "attacked" One of the easiest attacks is to use the violence that is often in such videos to get it age flagged. It's INCREDIBLY important to "backup" such media to re-upload in the case that an original source gets pulled.
If a downloader can't grab age restricted videos... It's pretty hobbled.
At least the preferences should have a "general" age entry, and option to auto disable age restrictions when it encounters it.