If every political candidate could be found to be senile solely on the basis of making gaffes, we would have hardly had a suitable candidate for president for the last few decades (probably longer). Is Joe Biden senile? I don't know. I'm not a medical professional, so I can't call it one way or the other. But you can't make a claim about one candidate based on a set of criteria without using the same criteria on the other candidate, even if you support the other candidate.
But I suppose the fact that most, if not all, of the senility claims (none of which are backed by any medical evidence) are being made by right-wing media outlets and Sanders supporters, is just a mere coincidence.