I would say Nintendo games are usually a bit "kiddie", but not really targeted toward any age group at all. Mario Galaxy looks like it'll be similar.
The game looks like it could be a classic. So far we've only seen one level, which takes place on an asteroid belt (you can jump from asteroid to asteroid). We don't know yet if other levels are like that or they're totally different. So as yet, it's hard to say if it'll worth buying or not. It should be out sometime next year.