In my case, I began to work on stroke efficiency in my late 30s, when I had already been practicing a less-efficient stroke for more than 20 years. My uneconomical stroke was a deeply ingrained habit. As long as I continued swimming, muscle memory would reinforce the old style. That gives you a blank slate on which to practice changes. This allows for dramatic improvements, often made nearly overnight, rather than incremental and reluctant changes, which are the norm.
Small pieces are easier to swallow.
We pick up skills easier by breaking a complex movement into manageable segments for practice. Because your stroke is made up of so many finely coordinated parts, it's virtually impossible to digest the whole thing. Then you simply reassemble these building blocks into a new, more efficient stroke. Instead of trial and error, it's trial and success. Because mini-skills can be mastered so quickly and easily, you begin practicing smooth moves right away.