Here is the study itself: https://journals.sagepub.com/doi/full/10.1177/1948550619835215
Furthermore, here are the study methods used: https://psidonline.isr.umich.edu/Guide/documents.aspx
Those who have studied psychology or sociology will probably look at this claim and ask "How do you measure happiness?" This is called operationalizing a variable. The outline of the study (the first link) explains how they measured happiness through self-reporting of one's life satisfaction.
"From 2009, the PSID has included a standard self-reported life satisfaction question phrased as follows: 'Please think about your life as a whole. How satisfied are you with it? Are you completely satisfied, very satisfied, somewhat satisfied, not very satisfied, or not at all satisfied?'" (from the first link)
If you want to get super technical, happiness and life satisfaction are technically two different things. However, the average reader isn't going to understand that well, so it makes sense that they would simplify the information for the general public to understand.
There are some graphs and charts included in the first link that display the information very well. The results reported in the article align with the results the study describes. While I'd consider this a simplified version of the truth, it's still true.