Sanders may well simply be ensuring that his preferred policies get another airing - if someone else steps up on a similar platform, he may well then step down in their favour.
I heard him on the radio this morning and he said that he wants to ensure that his message is heard. I don't think he's really expecting to win the nomination.
Also, Nancy Pelosi, the Speaker of the House, is 78, turning 79 this year. She's a very powerful woman in politics.