It's not that the media is inventing a narrative that Biden is old and senile, Biden has that reputation already due to his own behavior which is easily exploited by his oppenents, and even sympathetic media outlets have to make some concession because that perception is so widespread,
And hey, i don't think Biden is *totally gone* and we can't judge a person's cognitive ability by things like cringey soundbytes, but that's how the perceptions are shaped