Very true about age. I'm not sold his nearest competitors after Biden, Kamala and Beto have staying power outside their regions, I've seen Kamala and Beto talk and I don't quite see them finding a way to break off from the pack. I'm also not sold his age will be a factor once he starts delivering his message at rallies.
Bernie's rallies are by far the most exciting. If he hits the big blue states hard on Medicare for All and does it early, Biden is going to be weakened. Biden's somewhat slow, I hate to say low energy, speeches combined with his voting history can be used against him as well.
Biden won't have the presumptive nominee status and female candidate boost either.
Bernie will need key endorsements in closed primary states and have the excitement of people under ~40 going in.
If it comes down to Biden and Sanders, I think Sanders stands a chance to 'nickle and dime' Biden out in a lot of states grabbing 10 delegates here and 20 delegates there.
The polling on average has them nearly tied right now. And Bernie holds a rally advantage.
I think it's possible. He should reach a deal with Elizabeth Warren early if he runs.