Assume that professor rodgers had studied only people aged 65 or older and found that they experienced increasing levels of happiness in the last 20 years. suppose he had concluded that people under 65 years of age also experienced increasing levels of happiness during the same time period. he would have committed

The error of Overgeneralization. (Or overgeneralization fallacy)

Explanation:

This is concluding a belief based on a small sample size. It involves making a claim based on evidence that it just too limited. Essentially, you can't make a claim and assert that something is true if you have only an example or two as evidence.