If you are talking about the people who don't believe you that you were born in 2008, that's just an aspect about the internet society we have to live with. A lot of people here only have a stereotypical idea of how people of a particular age-group (even their own) behave, especially in the internet. It's a little price for our anonymity that strangers won't accept that you behave outside of your stereotype.

Keep in mind that most of them don't mean you any harm. Predators that lure their victims with a younger age or otherwise problematic scammers are a big problem of the internet and people would rather wrongfully accuse you of that than letting such a scenario play out.

While personally I didn't feel suspicious of you or have no reason to care, when I think about it, I can see that people get irritated by a person who is often highlighting that they are 18. If you really are 18, you probably didn't even notice that and it's natural. But maybe you can imagine that from another perspective, it may look different.