There is no critical period for learning music like there is with language. I highly recommend the book Guitar Zero by Gary Marcus. It's a scientific study on brain development by a professor who took up the guitar at 40. I found it so interesting. I've also learned from personal experience that age doesn't matter. I started playing the guitar at 45. I was taking lessons with my sons who were eight and 10 at the time. It's now three years later my boys will only practice about 15 minutes a day and that's not even every day of the week. I practice an hour or more every day and am much better than the kids. Age did not give them an advantage but practicing did give me one. They're still doing OK but they've got other interests sports, scouts etc. I just practice the guitar during their tennis lessons scout meetings and get more guitar time in. Don't lose hope just keep practicing!