https://www.c-span.org/video/?530006-1/president-biden-attends-community-engagement-event-lahaina-hawaii
fast forward to 19 min, or hell, start at 18 min... watch until he gets up... if we can't agree on this simple thing, where it is clear he is awake, no point really discussing anything here.
Out of curiosity, I watched this video paying special attention to the segment you suggested. Here are some things I observed. They are not intended to excuse anything, just trying to understand what might be going on with him and whether it is important to note or not.
While Biden did put his head down for a moment, it isn't clear to me that he fell asleep, or if he was simply looking downward.
The video did raise a couple of questions for me. I noticed he was breathing heavily (chest rising and falling) sometimes during the video, particularly at the point where it is not totally clear if he fell asleep. I also suspect he may have a deviated septum which can obstruct nasal breathing, because he appears to be a mouth breather. However, I found nothing confirming this. What I did learn is that he has sleep apnea. People with sleep apnea often don't get restful sleep so they are constantly tired. Neither a deviated septum nor sleep apnea are necessarily age-related health issues, meaning people of all ages can suffer from these conditions.
Biden's open mouth coupled with his slow movements and unusually small eyes don't exactly project alertness. One would have to look at videos of him when he was younger to better understand what of these facial characteristics, if any, are age related.
On a positive note, he doesn't appear agitated, but rather calm. This is unlike some people who are constantly upset and who as a result are red-faced as if they are about to suffer a seizure or heart failure.
I tend to be a bit fidgety while, Joe Biden is a bit of a sloth. If he were a young man, this might seem as him being calm natured, since he is old it tends to have a different connotation. The bottom line is different people will interpret his mannerisms in various ways.