Age does not have a significant impact on cognition for the average person until late the very old ages, and even then, cognition decline is not just "you're old, you're dumb". As we age, we do lose the ability to process brand new information as quickly(think trying to attain a new degree in school in a topic you've never studied), but you have a vast advantage in using accrued knowledge over younger individuals(think being able to apply similar skillsets between jobs). Biden is more than coherent and experienced to run the presidency. Now, with the convoluted mess that is politics and the lack of power a president appears to have in making change, his ability to change the country is probably not going to be very prominent. We might see some interesting things happen as congress is now democratic control, but that was the case under obama for a while and we didn't exactly propel into the space age. Only time will tell