Probably because a lot of 13 year olds THINK they know what they're talking about when they really don't. And I'm not specifically talking about you here, I don't even know you, I'm just speaking in general from what I've personally witnessed on this site. Of course, I will not judge people by their age, if a teenager gives good advice, I don't mind recognizing that. However, on more than one occasion, I have come across young teens who are very arrogant and cocky about their "opinions" and think they know everything when in reality, they don't.
I'm not saying that as a "dig" towards younger people, it's just something I have frequently noticed. That being said, I don't like making prejudgments about people so I will gladly listen to the point of view of a younger teen and take what they have to say into consideration, as long as they seem logical and rational. And at the same time, there are many older users here who also don't really know what they are talking about either. I think it's just easier for most people here to make these assumptions about the younger users BECAUSE they are so young.
So, yeah, I'm sure it sucks from your point of view but, that's just how it is. No one should be hateful towards you because of your age though and honestly, if someone makes personal attacks towards you because of your age instead of attempting to counter your argument, then you've already won the debate.