It is not common to develop a stutter at your age. However, what you describe does line up with some people's experience of what it is like to stutter. I think you addressed this in the post, but to clarify -- do you feel like you truly know the word you are about to say, and the issue is purely that you cannot bring your mouth/throat/breathing to say it? Or, is the issue that you know what you want to say, but cannot place/find the exact word in your mind?