Yeah, Biden has had a history of making gaffes, but usually quickly corrects them which Trump never does. I have never had to speak too much in public, but can tell anyone who has not that sometimes you cannot find the right word or the wrong words comes out inexplicably. It is best to correct the gaffe immediately if possible.
I too am concerned with the age of all the candidates being mentioned above and believe Warren to be the least affected by her age. If the majority of American voters truly want honesty and experience at the expense of an occasional gaffe then Biden has the best chance to beat Trump. I am confident that at his age Biden will put together the very best cabinet and WH staff and let them do their jobs. The same would go for Warren, I however am not sure about Bernie.
I tend agree that Warren seems to have been the least affected, mentally, by age. Trump, it has been leaked, is the laziest president ever, appearing in the oval office around 11:00 AM, after a morning of TV watching. Reagan wasn't known for long hours, either. I think all of the leading candidates have the mental ability to hold the job. "W" did. Not every president is going to be an intellectual blaze of glory like a Carter or an Obama. You could almost make the argument that intellect hampered Carter. Common sense is probably more important, that and the ability to select good people and let them do their jobs, as you've indicated with Biden.
Age and physical ability, I think is more of a concern. I realize that all of the candidates have stood up to the rigors of campaigning and that's a good sign. However, four - or eight - years is a long time, starting with age 79 or 80. Using myself as an example, you'd have a hard time finding anyone who'd state that I've lost much of, if any, of my mental ability - so far. I'll be 80 in a couple of months, so - older than any of the candidates. Now, I'm in better shape than 95% of the people I know who are my age. I just got through splitting and stacking three cords of green oak and hickory firewood. That's somewhere in the neighborhood of 12K lb. Still, I don't think I could guarantee that I could stand up to the demands of the office for, at a minimum, four years, or even, for that matter, guarantee that I, as a certainty, will be around for four years. It's just a function of mathematical probabilities. All in all, as you can see, I wish the leading candidates were younger...