That may be due to the stutter he had when he was younger.
Apparently you can correct the stutter in some cases, but there are still some lingering effects on the brain, including the occasional verbal gaffe (like searching for a word). It doesn't mean he is in mental decline, since it is something he has dealt with throughout his life.
Frankly I think in this election it may have helped him... Democrats have managed to steal away some senior voters from Trump (and they are often the most reliable demographic). Part of that may be because, well, Biden is "one of them".
I agree that it may not be mental decline, but whatever it is it doesn't look good for portraying strength and decisiveness.
None of us on this board are making any difference in the least by promoting one or the other. With that in mind, if possible, maybe being more objective would make these political conversations more interesting?