I'd love to learn to play the guitar. However, I haven't played another instrument. I've sung, and I can read a little music, but that's it. How hard will it be for me to learn the guitar as an adult?
By and large, adults have a harder time learning any instrument. That is not to say that you cannot. My statement is a generalization and everyone learns and dedicates themselves differently. Of all of the adult students I have taught in the past 25 years, per capita, less of them stick with it as opposed to the younger ones. Adults learn slower than kids but!!!! The ones that do learn tend to be more persistant. I only ask for a half hour a day practice from my beginner students. The adults that make the time to learn spend more time than I ask with the guitar in hand. Most....better then 1/2 of them don't have the actual time (or they say they don't) and they lose interest and the guitar just goes into a closet.
But wether you learn or not is mostly up to you and what you are willing to put into it. As a rule...most adults are too distracted by the world of reality and unrealistic expectations to stay with it. That's pretty much the bottom line.