Consider the sample size RJ. In the last 100 years there have been 15 instances where the Democratic Nominee was not a sitting president or a previous nominee. Of those 15, only one was over the age of 60 on election night. She won the popular vote. The other 14 went a combined 5* for 14.
*one of those 5 was stolen.
It’s an interesting historical note, but it’s not some ironclad political rule that a democrat over 60 can’t win the general election.
And, besides, Bernie ran for President in 2016, so this would not be his first time.