What are some differences between early adulthood and middle adulthood?
It depends somewhat on the person and the culture he or she is part of; for example, the development of a young person into a young adult, and then a middle aged adult might be somewhat different if comparing persons from China, and the United States. However, in general, for the "average" educated American, one could probably see these characteristics changing as the adult gets older. Young adulthood is often still a time of idealism, as well as a time spent getting one's feet on the ground. Leaving college and going out into the world can create an enormous amount of stress under the best of circumstances, and in this economy it cannot be easy. One's twenties are usually spent attempting to forge a career in either the field one trained for, or another one that the person stumbled, or lucked into. Changing jobs is much more common for today's generation of twenty and thirtysomethings, although as one approaches the thirties and forties, that may slow down as one may have a family and a mortgage to consider.
Middle age can be a time of stress, particularly for men who may experience the doubts and "what if's" that often accompany a mid-life crisis. This phenomenon seems to affect women less, although dealing with the man in mid-life crisis coupled with impending menopause may be a less than entertaining challenge. It used to be that middle age could at least be a time of reflecting on some semblance of financial stability and security and approaching retirement; however, the hijinks of the sub-prime lenders and the other factors that tanked the United States economy a few years ago have created a whole generation of folks who lost everything they had put away and are in the unenviable position of starting over in mid-life.