formative

Whatever gives shape to something else may be called formative: for example, the Grand Canyon is a product of the formative power of water, and the automobile was a huge formative influence on the design of America's cities. But it usually applies to some kind of shaping that isn't physical. An ambitious plan, for example, goes through a formative stage of development. The formative years of the U.S. included experimentation with various forms of government. And the most important formative experiences in our own lives tend to take place in the first 20 years or so.