Final answer:
In developmental research focusing on personality changes due to aging, 'Age' is the independent variable.
Step-by-step explanation:
In a developmental research where the focus is on changes in personality over a life-span due to aging, the independent variable is Age (A). Developmental psychology studies changes in various domains including physical, cognitive, and psychosocial development over the course of a person's life. The concept of lifespan development encompasses this broad range of areas and is concerned with how individuals change and grow from conception to death, with particular interest in how these changes occur across different stages of life, including old age. With an increasing elderly population, understanding the impact of age on factors like personality is vital.