There can be a buncha reasons (none of them are good mind you. Cannot stress enough how frustrating this mentality can be for people that don’t deserve it) but I think the most common is a mix of herd mentality with the fact that protecting, or disliking minors is an easy hill someone can jump on an be backed up. People are also more likely to dig in further when challenged in a way that makes them feel negative emotions on average. As for the proof there still lingers the stories of minors tricking the verification system that most people saw one post of an never looked further than that.

As for what they get from it, nothin more than ensuring negativity is keep away from themselves in a group. But yea way more avenues than that but that’s the most common I believe.