I don't really buy into the idea that he is becoming senile. I'm lucky enough to have had 4 grandparents last until my early twenties, with varying degrees of cognitive decline, including two with severe Alzheimers (one rapid, one slow). I don't see that in Biden's speech. What I do see a bit of is something like my dad's dyslexia. A lot of mix-ups in his speech are when he is reading off a teleprompter. I would be curious to hear what a speech therapist has to say about his issues.
I am also extremely reluctant to just go along with the armchair diagnosis when it's clearly being harnessed by Trump misinformation as well (see the 'manipulated media' video).
Have you seen other clips?
Seven seconds show serious speech sufferance
It's not that he forgot about very first sentence in the DoI, which is bad enough, it's the way he carried on through it.