Biden didn't have any obvious senior moments in his debate with Sanders last night -- he was a slick, practiced liar* and some viewers may consider that presidential.
Sanders attempted to point out that Biden favors cuts to Social Security, which Biden denied several times. Sanders told viewers, "go to YouTube" to see Biden's true position.
Naked Capitalism commenter John went to YouTube, and here are some links:
Joe Biden Talks About Cutting Social Security And Medicare. Update: This clip has since been deleted so here's another, from the Young Turks on December 2018. Cenk Uygur says he likes Biden (yuck) but not enough to vote for him -- this is before Biden became the presumptive candidate to go against Trump. Curious if Cenk changes his tune and gets behind Joe in the general. Biden is super creepy in these clips, whispering "social security" like it's some taboo thing, and saying it "can stay but needs adjustments" (that is, cuts or changes to the retirement age). Biden shares with many conservatives the belief that the US can't "afford" social benefits but can "afford" a huge military.
*Update 2: See Nina Illingworth's catalog of these falsehoods.