If the exit poll is right, Trump won seniors by 5. That's a 32 point error!! Senior samples have tended to skew female and that's why they are more Dem than they end up being, and that's been a problem for a while, but I don't even think response bias can fully explain this.
There was obviously an effort on the part of the media to scare seniors into voting Democrat by blaming Trump for the pandemic. Part of it I think was insane social desirability bias when it came to topics regarding the pandemic. Anybody who wanted the economy open or their schools open was accused of putting money before lives (despite other evidence that closing the economy lead to an unprecedented amount of suicides and overdoses, but we'll ignore that). They were scapegoated as not taking the pandemic seriously, not following the "experts" (you don't want to be against the experts do you?). I think this hit the hardest against seniors because they consume by far the most amount of cable and network news.
The other explanation I can resort to (but don't want to) is that it was something the overwhelming amount of Democrats (and therefore pollsters) wanted to happen, so they made it true in their polls. Then it became a consensus that seniors were abandoning Trump.
But regardless, the blue wave pushers ate it up. They believed it hook line and sinker. They didn't question the other age brackets or have any skepticism, because they liked what the poll showed.
An alternative explanation I heard (might have actually been somewhere on here) is that the seniors who answered polls were disproportionately Democratic/anti-Trump because they were the ones who were more likely to stay home out of virus fear.