
Well, I would say they are equally adult. I definitely wouldn't recommend either if your brother is really, really young, that's for sure; but I guess you know that if you saw the first one.
This one is basically longer and more fleshed out, but they do rush through some of the stuff from the original series. For instance, the first FMA anime had a certain character's (my favorite)'s death as kind of a halfway point. In Brotherhood, that happens really early on, I think in the first season. They also add more characters, and the composition and art style change a bit, I'm guessing to appear more like the manga.
I do like the way they fleshed it all out a bit more, and I like a lot of the new characters you see, so it's definitely worth checking out I think, cause while it still has the same essential plot points, it's very different from just plain FMA.