That article makes a fair point that there doesn't seem to be any evidence that being older hurts someone in the primaries. That's obviously true for Republicans who in the last 40 years have nominated Reagan at almost 70 (then the oldest new president) Dole and McCain at over 70 and Trump (the new oldest new president). Democrats have almost always nominated candidates in their 40s or 50s but that seems to be opening up a bit. Kerry was north of 60, Clinton was the frontrunner in 2007 when she turned 60 and last time, everyone ignored O'Malley in favor of Clinton at almost 70 and Sanders at 75 is the most popular politician among young voters in the country. (He even got booed for trying to play the young card.) Biden and Sanders would if elected turn 80 in their first term. But as long as you can hold your own on a debate stage, no one seems to care, which is good.
The only issue will be if he would attract enough African-Americans votes in states like MI, WI, PA, NC, SC, GA, MO, MD, FL and TX
I think many have mistaken Hillary's popularity with black voters for Sanders's unpopularity.
