It's interesting that you say that as if it were a sad thing (which I agree with kind of). Many people try so hard to grow up before they are actually mature (I was one of them; see username :/ ). Itachi's maturity was able to keep up with the burden of his experiences 
Or something like that...
So, your response essentially caused me to take 5 or so minutes out of my life to quickly type some walls of text and essentially go off on a tangent. Be proud. I guess I got sentimental. Lol
Well, I can somewhat relate since I grew up pretty fast (imo) as well. Mainly through choice and desire to progress.
But in terms of the character, I find it both sad and admirable.
Sad= The era he grew up in. The war ended when he was 4. Post-conflict, he should have lived a more care-free life. However, he could not. He basically missed out on his childhood.
We always saw the kids of Naruto's generation, even some kids of the war era (Hashirama and Madara did have times to have fun, though they were few) having joyous times and being relatively care-free.
Itachi was stressed since he was 4. He was destined to live a harsh life in a way, since he was supposed to lead the Uchiha clan. Everyone set very high expectations for him. Good thing is that he met them and even exceeded them. Though this still takes a toll on an individual. Since he was ten, he had to be a double spy- his heart being torn between his family and the shinobi world as a whole. Then, when he's 13 he has to make a choice on whether he carries out the annihilation or not... this brings great agony to him. He also basically left his future (his own hopes and dreams... of which we as an audience can only speculate) on that night, then dies at the age of 21.
He was a pretty young guy who didn't get to experience what it's like to be a kid. He always had great responsibility and as a result had no better way than to grow up quickly.
Admirable= He did not complain. He didn't even mind. He chose a harsh path due to his kind nature. He noticed flaws along the way and encountered a few obstacles, but he did not give up and instead chose to adapt. I think that's great.
Also, I've read plenty of literary works. A copious amount. Anytime an author would make a crack at going for a contrast with a character, it would usually fail. The character would seem nonsensical, poorly written, etc. However, with Itachi it worked really well and I'm greatly impressed with this aspect of Kishimoto's work. It likely worked since the author bothered to provide in-depth reasons to each act this character committed and he's ideal for the general setting of this story. That general setting being a shinobi world, not just the Shonen archetype.
Not only that, but Itachi is a character that can garner sympathy from people of nearly all walks of life in some way or another. He's realistic in that he was a good individual actually having to deal with a conflict, in a story where every other protagonist could easily find an alternative with little effort. The latter should not be possible in a story as dark as the Naruto series, but it was in order to push typical inspiration themes to the predominantly child audience.
Also, his relationships are both heartwarming and plausible within our world in their most basic sense. Whether that be a sibling bond that is endearing to the fandom as a whole (since even people without siblings know what it's like to want to protect someone dear to them), or parents (since most know that it hurts to disagree with a guardian on any issue, but they still love them regardless of the nuance), or friendship (there are friends out there that people hold so dear that they might as well be family, and we would do mostly anything to honor them and keep a promise).
Another good point is that though Itachi's bonds with others are deep, they are not petty (most relationships in this manga were...no offense pairing fans), obsessive (he could let go of things even if he cared a considerable amount), or exaggerated (most friendships were too cartoonish... Gai's and Kakashi's is nice in theory but a good example of that. Grown men won't be acting like that. It was done for entertainment purposes for the younger audience. Whereas Itachi's friendships, like that with Shisui, were serious and impactful even with having a very short amount of panel time. Not only that, but even with such a friendship held in high regard, they could tease each other- shown in the game storyline Kishimoto approved.)
Long story short- He's one of my favorite characters of any genre and definitely my favorite of the Naruto series. Now that I think about it, the majority of the lessons I learned from this series came from his character. He was also the main reason I kept up with the series (since Kishimoto kept mentioning him even after death and adding on to his characterization) other than of course wanting to see where Team 7 would end up. Years down the line when I've forgotten most of the Naruto series plot line, I'll likely remember this character fondly.
About his fandom- Outside of the funny trolls, it was one of the best I've seen.
Why? Because of the way it sticks together. Any character can be bashed, but no one can say a word about this character before getting shut down. Which makes sense, since due to how well he was set-up, bash is not warranted.
Why? It's real. Most of the other Naruto fandoms are just bandwagon. Itachi has had a strong fandom since 2002, with it increasing. Even with more powerful characters being shown, proving people liked him for the beautiful characterization.
Why? Have you seen the AMVs? Most of the best AMVs I've seen are about him. In terms of the music and editing that is.
Why? The fanart is unbelievable and can tear at one's heart strings. There are many talented artists in this fanbase.
Why? The level of analysis. I have not seen any other anime or manga character being discussed to this degree and in such depth. Most of the best character analysis threads on NB are about him. So, yes. This fanbase is pretty decent. Too bad that people are as narrow-minded as to allow trolls to taint what it's really about (interesting discussion) and are content with letting the few represent the majority.
