Who decided it was only for kids? With many popular games such as GTA being given an 18+ rated title (in the UK), clearly it's not just kids buying them. Kind of a stupid thread title if you ask me. Sure, many people under 18 play GTA, but the point is if the government and legal bodies thought games consoles were only for kids, they wouldn't allow games not for kids to be released full stop.
The statement in the OP, and the thread title appear in no way linked, where has the 'only for kids' thing come from? And the reasons for console gaming and certainly nothing to do with age. Consoles are quicker, easier, certainly cheaper and easier to maintain and update without losing the core experience from many titles. Sure PCs can be more powerful, but the money involved just isn't justified unless you have plenty to spare, or really love PC gaming specifically.