A 60 year old is considered old. So is an 87 year old. But they are a generation apart and a 60 year old doesn't want to be called by the same term that you'd use for his father.
A lot of this is going to be regional. Where I live, one would be the elderly parent of a senior. The older one, in the US, might have a pension. The younger one might never have one, because they don't come from the government, unless the person worked for the government.
So it should really come down to Duolingo accepting all terms for it, but you won't find one that everyone feels comfortable with.
I used "senior" to stay on D's safe side but I don't think it's the word of choice here. I'd prefer elderly person, even old person (I'm one). But let's not forget "senior" has other meaning aside from business in the US it means a student in the final year of HS or College.
Yes but not in this sense, and it would be a nasty remark that you would rather reserve for children. "Dieser Junge kann einfach nicht hören!" "Jetzt kann der Junge (endlich) wieder hören." I hope you would not tend to tell this about a senior. In the lecture sense the sentence would miss an object. You could say: "Der Senior kann sie noch einmal hören." (sie = die Vorlesung or ihn=den Vortrag). With repetitions you would preferably use "noch einmal" instead of "wieder". Also you would be easier to understand with "Der Senior kann sie noch einmal besuchen."
I would like to repeat my comment that "Senior" in German would mostly be used in an impersonal context. As in "Senioren haben 30% Rabatt.", "Senioren dürfen in der ersten Reihe sitzen." If you are talking relating to a person, "Senior" would often be considered impolite. I am almost a senior myself, and I would not like to be called "Der Senior" even in a few years from now. "Der ältere Herr kann wieder hören." / "Die ältere Dame kann wieder hören" (them having got a hearing aid) would be much better. But beware: "der alte Mann" / "die alte Frau" is also impolite.
This was a hard one as I know that a phrase often used to finish a telephone call is 'auf wieder hören'. I expected an idiom as the accepted solution so was surprised that my literal translation was ok. I am very senior, beyond caring how we are addressed bar just one horrible term: 'Young Man!'