Anyone can be brainwashed, just like anyone can fall prey to a cult - be it religious, secular, business, political, sports, social, whatever. It doesn't matter whether one is dumb or intelligent, crude or sophisticated, naive or worldly, uneducated or highly educated, young or old... anyone can fall prey to even the simplest of mind control techniques - and not have a clue until someone else is able to point it out, usually following some sort of reverse process that undoes the process of mis-information they'd been led down.
Moreover, I'll clue you into some of those most susceptible to it - you identified them - those who haven't plied any critical thought to the beliefs they've been fed - largely because they were disposed to espouse those beliefs in the first place - and THAT largely because they were fed the information at a time, and in circumstances where they had no reason to question them. Predisposition.
Something else you should know - because I gather from your comment (correct me if I'm wrong), that you have a conception of brainwashing or mind-control (however one describes it) that imagines people wandering around (and understand, I'm exaggerating a bit here, and that for effect) in some sort of glassy-eyed daze or trance, only to wake at the snap of the controller's fingers to do their bidding. That is the Hollywood perception of mind-control. Mind control techniques vary immensely - from the very physical (violence, threats of violence, abuse, torture, etc.) to the very subtle forms of manipulation by one human on another.
All forms target something the victim deeply desires, which is why they are in fact so successful. Security (emotional, physical, psychological, religious, economical, etc.), companionship, a sense of belonging, purpose in life, meaning in life, release from any of a number of forms of 'captivity' - and so on. It doesn't matter how critical a thinker someone is in one or more aspects of their life, there's usually other aspects in which they're vulnerable, where even the best critical thinkers fail to ply that discipline.
Actually, what Holder is espousing in that video is one of the highest forms of insult - he actually believes it will work - and frankly, he's largely right - which is what makes it so insidious. He knows that with a steady, repetitious diet of "his" information pummeling people from every direction and every source of "credibility" that he can sway his target audience to his way of thinking, to basically do his bidding - his bidding in this case being to vote, to voice "their" opinion, to organize, to protest, to write their editors, congressmen, to report others, etc. and thus eventually effect his goal of getting guns out of the hands of law-abiding American citizens. That's not hyperbole or exaggeration - that is his goal; and as you rightfully point out, it's arrogance of the worst kind - plied in a seemingly innocuous context - or more aptly, pretext of 'protecting' youths on the streets of DC (never mind the information campaign is going to everyone...).
Well, I have to agree. It is condescending, and arrogant, and frankly not a tad tyrannical. But as I said above, what makes the methods he outlines above work, is in fact a target audience largely pre-disposed to accept the message uncritically - whether their motive is fealty for the party, a desire to see an end to violence, or feeding a belief already inculcated - the arrogance of the methodology is that, while being utterly manipulative, it is a fundamentally dishonest methodology as well - dishonest in its approach, dishonest in its presentation, and dishonest in its ultimate purpose.