Biden isn't senile. I'm so sick of people that actually make that argument. Firstly, "senile" is an outdated and now pejorative term for a person with Alzheimer's, so I wouldn't use the term regardless.
Most people (including you) have claimed that Biden is in mental decline due to occasional gaffes and memory lapses. This, coupled with Biden's advanced age, does not indicate Alzheimer's. Almost 100% of the time, Biden gets through all speeches and statements without any gaffes, forgetfulness, or even stuttering. I've found that those who claim Biden is "senile" base their opinion mostly off cherry-picked viral moments and gaffe compilations rather than actually watching the man speak. Biden's response to every time he's been confronted about his supposed poor mental acuity is "watch me". He's exactly correct; if you actually watched him, you would see that he's perfectly capable of performing his presidential duties and can think on his feet. Those who claim he is in decline do so becomes they aren't watching him, besides only when he's at his worst. And let's be honest here: a large chunk of his "gaffes" and "memory lapses" are an intuitive defensive mechanism to avoid a stutter. And for the ones that aren't, who cares? Anyone with this job is perpetually under high stress, constantly delivering all sorts of addresses, so the likelihood for occasional gaffes is significantly higher than the average person. All presidents (at least the ones in recent history) have made gaffes and memory lapses. It's just that Biden is being accused of having Alzheimer's when he does so because his old age makes that argument all the more convenient, which isn't very fair.
Besides, a president in mental decline would be a major threat to national security and thus, would not even be allowed. Suppose Biden genuinely did succumb to Alzheimer's and gradually became less able to remember information, deliver speeches, and perform other duties. How would both his own staff and Congress permit this? Such a thing could easily cause the president to make rash decisions and/or divulge information that he shouldn't. Furthermore, Alzheimer's is far more than just memory loss: if Biden had it, he'd also be displaying delusions/psychiatric symptoms, random mood swings, diminished physical abilities, among other symptoms, all of which Biden does not display.
Anyway, it's more likely that Biden mishandled the classified documents because he was irresponsible, rather than him not being capable of remembering where they are. And if he did genuinely forget- which is no excuse of course- it doesn't signify mental decline.