It was much easier to do “deep fakes” prior to the 19th century, when they used artists (with very corruptible human intelligence).
If you were clever, it could be with photography,
Modern ones tend to be obviously fake, and the ease of doing them means no one will trust a photo.
As for Biden, he appears to be as mentally and physically able as most people his age. Of course, most people “throw in the towel” prior to turning 70, and probably the country would be better or if Trump and Biden were content with being retired.