Am I too old to still like Minecraft? And why do people hate it?

No, you can like whatever games you want. Don't let the people at school define what games you should like and not like. You are not too old at all.
I personally love minecraft and I do not think it is kiddish.(I don't even know why anyone wouldn't like it!) In your grade, no offence to you, but there are young adults blossoming into full hipster douschebaggery and a side effect of that is hating everything that is mainstream or popular at the time.
Just ignore your classmates. Like whatever games you want and do not worry about what others think of it.

I know adults who are into it. Kids are always saying something sucks to try and seem like they are "too cool" for something or to do something.

It takes some time, but I've come to realize that you like what you like, and life's too short to pretend you don't enjoy them. The real losers in life are people who spend their time trashing other people for having interests they don't understand or share.

No do want you want. People that are gonna judge you are lame.

You're not too old to play video games. I don't play Minecraft simply because I don't understand the game itself, and I don't really have much time to 'study' about it since I don't have much time to play video games either.
Maybe people hate something when they don't fully understand it.

Listen, I'm 16 and I LOVE Minecraft. As a matter of fact, that game was not intended for little kids. A huge percentage of MC players are your age and older. Those people that say they hate it are crazy.