Same here. I think that's a shortcoming of the Duo program. I think they should be more concerned about the target language. Especially since English has such wide and variable rules of what's common or acceptable depending on what region you are from. Leave the English alone, Duo! If I wanted English lessons, I'd take them.
I know this sounds harsh, I don't mean to be offensive, but the course is indeed has "For English speakers" in the title. That means there's an assumption that learners already know English well enough to even be familiar with idioms and some regional variations. If not (as is my case), it's an opportunity to learn more English, which is great!
I'm English speaking and have lived in many different areas but I cannot think of any circumstance in which I would say "The seniors fall". However, it is useful to know that "Senioren" means elderly people. I might want to say something like, "Elderly people fall more often than younger people." I would like to know if Germans ever say, "Die Senioren fallen" and the context.