I'm 19 years old and people always infantilize me.
That's a little weird. Why do you think that they do that?
I don't think people are being mean. I think they're treating me like a child
Yeah, that really does sound like it may be a little bit annoying. You should try having a conversation with them so they know that it hurts you.
I'm going to ignore them. They deserve it.
I understand it's frustrating not being treated in age properly, but it's not a good idea to ignore them for it either. They might have their reasons to do this, so why not try talking to them and figuring it out?