Timothy, how about exploring the types of things that go on which will affect learning. I believe that a lot has to do with attitude and approach.
- Many adults who have never had music lessons before approach them with preconceptions which prevent them from responding directly to things. Conceptualizing is an advantage, but it can also be a barrier to direct experience: you absorb what you think you should be hearing rather than what you do hear (and feel etc.) Knowledge of this can circumvent it.
- Our training in school academically, especially in higher education (many adult students are those who can afford lessons) forms a barrier, because a beginner should not work like a university student. We're in a world of counting to three while getting a handle on our limbs and getting those bouncy black and white things to make interesting noises. The more physical and direct with no preconception of what to expect, the better. Again, this is habit.
- Adults may start lessons with an idea of what pieces they want to be taught, how those pieces ought to sound, and maybe how they think they should be taught, and (unconsciously) how they should be listening to their teacher. They may go home and practice how they think it should be done, paying attention to what they think needs attention. This is habit and attitude, rather than ability or potential.
- There is an atmosphere that the older student should not take himself seriously, this being reinforced by society, a kind of embarrassment or shyness that will interfere with success. Also, being taken in by the illusion of the trained artist that all is done with ease and "talent" so that when that fluency isn't instantly there, the older student will think he is lacking something. Small children's studies are taken seriously by society, and being clumsy novices is a way of life for them. They know they are "growing up".
- In the past we saw attitudes by teachers that adults should not be taken seriously, any kind of music could be thrown at them. The type of aimless teaching that I saw recommended a decade ago would make any student of any age fail. This seems to have turned around.
- If you compare "adult methods" with regular children methods written by the same publisher, you'll find that the regular method aims to teach numerous skills and give various musical experiences, while the "adult" books ride the surface, catering for a wish to go fast and play well known tunes. If only one group is given solid foundations of musicianship, is it surprising that the results are different?
- I suspect that our sophistication works against us. Just because I understand something intellectually does not mean that I have it in my body and my ears. But will teachers give adults the same richness of experience that children get, or will we be immersed in concepts and sophistication too early? (Noting my repeated emphasis of experiences of sophisticated things). Do we know how to practice, and is this taught?
That is quite a list of things that can get in the way of progress in learning to play an instrument. Unless they are addressed, you cannot know what someone's potential actually is. Or put another way, statements about teachers avoiding adults and possible failures have to take possible causes into account.