My uncle is a private practice neurosurgeon and is convinced that Biden has hydrocephalus (pressure on his fornix), which prevents him from recalling/remembering recent events. His gait shows signs of it too, having his feet stay very close to the floor as if his feet are magnetic when he walks. And obviously his overall quick decline cognitively.

Edit: Removed "shuffling" from the gait sentence above to put emphasis on the low foot rise in his gait and the floor appearing magnetic with his feet. More accurate to show the difference between old man shuffle vs. hydrocephalus.