It turns out it's not the "obvious" better choice they wanted it to be. So looking into it is definitely interesting.
Where are you pulling these stats from? Do you have any references or do you just keep misrepresenting your personal observations as facts?
I sure do see my 40-60 age group friends and acquaintances going for more diet soda than regular soda. So I find it hard to believe that diet soda is not a common thing in the 40-60 age group.
It is frustrating that for someone who posted strong criticism against a research paper, you keep making bold claims without presenting any references or evidence!
DIY at least you'll know I'm not referencing selectively.
For example, when I did search, I came across this source: https://ec.europa.eu/eurostat/web/products-eurostat-news/-/d.... However, I'm not convinced it supports your point that this trend is 'common' but not in the 40–60 age group. The data show that 5% of people aged 40–60 use it, compared to 12% in the under-40 group. That doesn't strike me as a large enough gap to justify saying it's common in one group but not in the other.