Short answer: I started out knowing nothing, but with three years of hard work, I've doubled my knowledgeOriginally Posted by ArchieComic Fan;3141125;
Long answer: When I started taking lessons three years ago (May of '05), I knew very little about music. I was familiar with the keyboard layout, a handful of chords that I picked up (who knows where ) and I could sort of read the treble clef. Reading the music came fairly easily to me, but playing with any kind of coordination between my hands was a back-breaking effort. My first teacher had the patience of a saint. She had been teaching for fifty years, and had seen it all (but hadn't heard it all; one time, probably about my third or fourth lesson, I told her that I didn't do my homework because Gustav and Bjorn (who I had mentioned the previous lesson, so she knew they were BIG) had eaten my piano Another time, I was getting frustrated and told her that if God had meant for me to play the piano, He'd have given me 88 fingers). We went through a couple of books in one of the instructional series, but she also let me plink away at some classical and operatic stuff to keep me enthused. My current teacher doesn't have me on a specific learning method, but had done a good job of selecting collections that keep me interested and learning. She's also been quite good at teaching me how to practice (to make more efficient use of my practice time, how to learn difficult passages (well, difficult for me and maybe the 4-year-old that she teaches.) I'm usually working on three pieces at any one time, but while she's on vacation, I'm supposed to be working on six. I practice fairly diligently, and it pays off. I sort of figured that I'd take lessons for five years, but that isn't based on anything. It impresses the heck out of me when someone sits down with a new piece of music and starts playing, but I doubt that I'll ever get to that point (I do pretty well with the right-hand part if it's a piece that I recognize, especially if I know the lyrics.) If I remember correctly, by the end of the first year, I could play a melody and chord accompaniment (ie, from a lead sheet) fairly well.
Another short answer: Both of my teachers told me that adults have more trouble than kids when it comes to learning to play with both hands, but that the adult grasp the concepts better and have much better focus, so neither teacher feels that it is a big impediment to start as an adult. My oldest sister (just on the high side of sixty plans to take lessons this fall.