I’m 22, 23 next month. The reason you’re being labeled as a child is the way you type. The vibes people are getting from your messages/posts do convey a pre-teen person. Tone down the exclamation points and capitalize the beginning of sentences. Emotion Faces should probably be reserved for chats with friends or family as on the internet they are tied to younger people. Here’s a re-written version of your post as an example:

“Whenever I post or talk to people on an online forum or something, they think that I’m a child or child-like. But I’m not, I am a 20 y/o. It’s fine if they think that, but I think it’s disrespectful.”