There is no “acting your age”, there are only stereotypes, expectations and some people who are afraid acting out of those.

Do I know why that happened to you? No, just guessing.

What I do know though: if it was like some replies here suggest, that it’s all due to IT folks not playing well with others, then forums like stack overflow wouldn’t exist.

What I also know: I’ve been to a lot of forums, not all IT related, and I met quite a few people online who just love to be rude, regardless of the topic.

So if I had to guess why, I’d say because they are assholes, not because it was an IT forum.