This title just came out so there's not alot of feedback yet...Besides the one review someone posted here the game site IGN has also given this a fairly lengthy, not to mention glowing, review....so maybe check that out.
My guess is that if the 12 year old is able to tackle recipes in some of the bestselling cookbooks then I'm thinking this title is probably on a par with those; Probably depends on the individual and how advanced they are reading-wise.
I would say that they're not too adult. A few of the recipes would need adult supervision, of course, but the majority are simple enough, and any that aren't would probably only need a little guidance. The majority of the recipes are quite easy and the game explains a lot of terms and supplies details where necessary, to better explain certain parts if needed.
I just bought this on Friday and have found many recipes that I personally would say are very suitable for children.