Lifespan developmental studies are based on the idea that all parts that take place in an individual's life, or lifespan, are a sort of continuum of things that are influenced by things that have occurred in the individual's past. As a result, things will continue to affect events in the future.
In not so many words, while life begins with infancy, is nourished during childhood, and continues through adolescence, the events that take place in those key developmental phases may be the makers and/or breakers of future behaviors.
When people reach adulthood it does not mean that lifespan development is over. On the contrary, it means that the issues from the past must be resolved, or should be easier to resolve now that the individual has more knowledge and experience in problem solving.
However, when the issues do not become resolved, they stunt the individual and even become worse with time. This is why it is so imperative that people take the time to meditate, analyze where life is taking them, and make correlations as to why things happen to them. It may be that some behaviors shut people out of your life, or that you aspire at things that you are not really ready for. Read more on lifespan development on eNotes.