Teens are still young and impressionable, they can be easily affected by the opinions and beliefs of those around them (teachers and who not), not to mention their brains are still developing.
As are most adults. We just thrust that idea onto the younger generations because it makes us look like we're more wise and experienced in the world.
Yes, our brains are technically finished developing after a certain age, but that doesn't make us any less impressionable or un-swayed by the opinions of others than those teens we're trying to discredit with that logic.
You forget that a majority of people out there base their opinions off of what their news source, favorite celebrity/brand, religious institution/figure, neighbors/friends, or political party believes. If the news for example paints someone as a terrorist, then that's how people see them, even if those people had never met them before.
Just look at how it's affected politics in the recent election. One side painted Hilary Clinton as a mustache twirling villain, so people saw her as such, even though a majority of those people don't actually know her. But because of how she was portrayed on the various outlets, people automatically hopped into that bandwagon.
Now, I'm not saying that she's not a bad person or that everything that was said about her is necessarily untrue, but I am saying that because people we don't even know SAID she was a bad guy, enough people believed it to where it affected the elections.
Likewise with Trump, because we were told by people we don't even know that he had outside help in the elections, it's something that's become instantly believed. It may be true, and it may not be true, but because sources that we believe even though most of us don't personally know us SAID it was true, a majority of people believe it.
Point being, if the right source comes along and urges us to believe something, our common sense and rational thinking go out the window and we believe them. Why? Because even if we don't know that person telling us it, we still believe them for no apparent reason other than reasons such as them just "looking" trustworthy or because they say things that resonate with you, even if you don't know if they believe it or not.
Age doesn't matter when it comes to being impressionable or easily swayed. If the right person or source comes along, we're all impressionable to a degree.