Dr. spock is testing the same individuals repeatedly over time as part of a research study, while dr. smooth is comparing the performance of different people of various ages at the same time. dr. spock is using a research design, while dr. smooth is using a research design.

I believe the answer is: longitudinal; cross-sectional
Longitudinal study is initiated if the researchers want to collect long term data from the subject as over  time (as they grow up).
Cross-sectional study on the other hand is initiated if the researchers want to know whether a certain things only affect a specific population.