It's mostly because 11 year olds are... Well, immature, to put it nicely. I'm not saying you're stupid, but on the internet, especially on forums, you'll be mixing with people who are all mostly much older than you, and during puberty you mature and grow up so fast. I mean, I am not the same person that I was when I was 16, and that was only a year ago, you know? We all change and grow up, we're still learning and trying to find our place in the world, it's totally normal to go through "silly phases", I suppose. And even now, I'm still growing up. I still have a way to go, and I'm 17. So basically, you are mixing with people of all ages, and most people are older than you = therefore you are seen as being less mature because you've had less life experiences as those older than you so. You might sometimes say things which might seem "stupid" to those who are older than you, and that's just because... You're just not as mature. And I really mean that in a nice way! You will understand. You will understand when you get older. I remember I totally felt the same way as you when I was your age, people used to make fun of me on websites because I was a "noob" lmao, but now when I look back on it, I'm like yeah, I can see why people thought I was like that.
And to be honest... I always see kids on this site say "I'm mature for my age", but the truth is... You aren't. Okay, you may act a bit more grown-up for an 11 year old, you can type with correct grammar and such and your posts aren't ridiculous like some 11 year olds, but you still have a lot of growing up to do. You still have puberty to go through (yikes - good luck with that! ^^), you still have a lot of life experiences to... You know... Experience, I suppose. And through those experiences, you'll mature and develop as a person. You can't be "mature for your age", you can't have the intelligence and maturity of a 20 year old when you're 11, because you won't have experienced the same things as a 20 year old would have. I hope you understand what I mean.
Unfortunately, people on the internet are mean and spiteful so, they may not be very understanding or very nice to you because you ARE 11, and they may be impatient or annoyed by you. Sometimes people try to pick on someone who is an easy target, so they use age, for example, because it's a defining characteristic of your personality, and they'll make fun of you for that and make you feel vulnerable. The most mature thing to do would to just not care or respond to what these people have to say. I mean, clearly there are some people on this forum MUCH older than you who have a bit of growing up to do, lol, if they make you feel that way.