If you have no interest on anything other than hobby playing, you can throw a large amount of theory out, and by that I don't mean ignore it but I mean you don't have to memorize the relative minor of a G major scale (G major scale - what's that?!), but you have to be aware it exists and know the concept at the very least of music theory which will greatly aid your playing and reading of music.
oh now addod... you did NOT just say that when you know I am lurking about today and that I love my theory. lolactually when it comes to adult beginners I am always very curious to know what exactly has inspired them to learn the piano. to the OP: Is there a reason why are you interested in this now? do you have a musical family? when did you first think you might someday want to play the piano? who was the person who first introduced you to the piano?Is there a specific piece of music you have always wanted to play? Have you ever seen a live piano concert? who is your favorite pianist?have you recently had a major life event... lose someone? or maybe just changed directions?