no, our DS was 13 and while we didn't dress-up or anything, he loved the free candy and the fact that you could walk on most rides without waiting in line.
If your 12 yo likes candy and riding SM, BTMRR, HM, etc., then he'll love MNSSHP.
There were a lot of kids out of costume, so he won't have to worry about dressing up. We saw many kids 12+ at MNSSHP last Oct. ... and they all looked like they were having an amazing time! I say go for it!
You're never too old to enjoy MNSSHP. I was there with a good friend of mine last October. She was 29 and I'm 33. There was this moment when we were sitting on the Liberty Belle, in our costumes, waiting for the storytelling to begin. Well, we were sitting on the deck of the boat indian style and had our bags of candy on the ground and we started trading candy. "I'll give you my laffy taffy is you give me the bit-o-honey." We both realized how "kid like" we must've appeared at that moment and just started laughing. It would've been great to have a picture of it.
We went when my daughter very almost 14. While she didn't share her younger brother's enthusiam for villians in costumes ,
all 7 dwarfs together posing for pictures with guests or free candy,
she loved the chance to ride on Space Mountain about 10 times!!