Developmentalists often focus on one age group in the life span and they all also consider:

The answer is: what happened before and then what will happen after each period.

Developmentalists usually focus their works in order to determine how and why individuals change over the course of their life.

In order to find out this answer, They tend to conduct their study in a long-term period, in order to compare several factors that might influence the change such as age, cognitive development, and social interactions.