Final answer:
A study evaluating behavioral changes with age by comparing different age groups at once is called a cross-sectional study.
Step-by-step explanation:
A research study that assesses changes in behavior related to age by examining different groups of individuals, each group representing a different age, is called a cross-sectional study. This type of study is an observational approach where a researcher compares multiple segments of the population simultaneously. For example, in a study on dietary habits, the researcher might compare the eating patterns of groups of 20-year-old individuals to those of 30-year-olds and 40-year-olds, identifying differences and patterns that might be linked to age. However, such a study may be limited by generational differences that are not related to age itself, but rather to the unique social and cultural experiences of each age group.