As a long time professional musician and teacher, I can tell you that without a doubt, almost anyone can learn to play an instrument well enough to please themselves, and in a relatively short period of time. Short enough to keep from getting bored and quiting anyway.
Unfortunately, some students are like religious people in the fact that they have faith. Faith that they cannot learn an instrument or that they are tone deaf, etc. Not to say that religious people are lacking in talent. On the contrary. They have some of the most beautiful singers and musicians on the planet.
The problem is with the teachers in school. The problem is also with the accepted and standard theory of music. You have to understand the principals of the society of teachers. They are trying to help you play music with other people. They teach you music that needs to be played exactly as it was written. It's the easiest way to teach large groups of people how to communicate and work together to form an orchestra and follow the conductor. If you plan to be a professional musician and you have the goals and dreams and really plan on giving it everything you've got, then first learn to play by ear for a little while, then learn music theory and to read music. It will be invaluable and it is the reason it is taught the way it is. As if every child wants to be nothing more than a musician.
It is the conductor's job to bring out the heart and soul of a piece of music by his orchestra. Teaching to play by ear is seldom ever taught, and never in school. There are as many ways to teach music as there are types of music to learn.
Now to be sure, there are people out there with bad ears. It has to do with the inner ear and how it hears tones. Some people cannot distinguish between to different tones. My experience is that most people can distinguish these tones, but have no training in how. Sufficient training can usually teach most people how.
Music is about emotion. It is also a form of communication and is certainly more effective in conveying what a person is trying to say than even a poem by itself can possibly do. It's like the difference between reading an email and watching Beyonce do her thing. She puts her whole body into the communication.
I did have one student that wanted to play so very badly. I never learned how to teach him though. He couldn't remember a single chord I ever taught him on the guitar come next lesson, even though he would show up with bloody fingers from all the practice he did. I have to admit that his IQ was barely above a chimps, but I have known seriously retarded students who learned to play with eloquences.
There was also a band member of mine one time who could never remember what we practiced the week before. I don't mean that he couldn't remember how a song went, I mean that he couldn't even remember trying to play certain songs. No recollection of the practice whatsoever.
Never let a music teacher tell you you aren't suited for learning ANYTHING. When I started a school with a music program, I decided to take band. After a week of evaluation the teacher decided to put my on the cymbals. That's only because when he told me I was too far behind and would never be able to be a good musician, I stubbornly refused his advice. He had no choice but to let me join, so he taught me the basic symbol for timing and where I was suppose to slap my cymbals together. I had already been playing guitar for two years by ear, but he thought I could not be a musician without reading music.
Well, Mr. Teacher, I went on to learn to play 11 instruments, learn them all well and in most all styles of music. I have won awards with bands I have been in (Budweiser Battle of the Bands twice), opened for several big time acts such as Sammy Kershaw, Doug Kershaw, Alan Jackson, Exile, Rhett Akins, Big House, The Bellamy Brothers, T.G. Shephard, and many others. I got to party with many of them and hear their tales of the road, and tell them a few of mine as well. (18 years on the road myself.) I have won karaoke contests, and song writer contests and have written over 300 songs myself, all because I never let some grade school teacher tell me what I could accomplish. So don't you either! If I can teach a kid with an IQ of 90 how to play some of Eddie Van Halen's solos, then you can certainly learn how too.
Its a matter of how much time you practice and having the right tools to learn.
For most students, I teach this way:
First, you learn to have rhythm. Learn basic snare drum rudiments. When you've mastered several of these we move on to the piano. Learn the chords and how they're structured, plus quite a few other things. Then you move on to the instrument of your choice and play with it for a few weeks. Get to really know the thing. Then learn basic chords on it or the fingering of a solo instrument.
After that you can start learning basic songs. Maybe five or six lessons tops, and you very well might be on your way to doing something you can really be proud of. If you're lazy though, hang it up. If you get bored too easily, there are other ways of teaching that can show you how, one song at a time, to play. I don't recommend this method though, because you will learn all kinds of bad habits that will stunt your growth and keep you from ever really becoming truly great without unlearning these habits after a period of time.
If you love music, you can learn how to create it yourself with a little effort and time.
Tool Hand Luke