http://thehill.com/homenews/campaign/404644-john-kerry-wont-rule-out-2020-white-house-runGiven the discussion of Biden and Sanders, John Kerry seems like a fine idea.
In fact, why not put them all in - they are each of them likelier to be better prepared than anybody on the other side who is available (and healthy enough).
2020 Democratic candidates for nomination:
Hillary Clinton - October 26, 1947 (age 70 years)
Al Gore - March 31, 1948 (age 70 years)
John Kerry - December 11, 1943 (age 74 years)
Joe Biden - November 20, 1942 (age 75 years)
Jessie Jackson - October 8, 1941 (age 76 years)
Bernie Sanders - September 8, 1941 (age 76 years)
Mike Dukakis - November 3, 1933 (age 84 years)
Walter Mondale - January 5, 1928 (age 90 years)
Jimmy Carter - October 1, 1924 (age 93 years)
Or some fresh blood like:
Jeanne Shaheen - January 28, 1947 (age 71 years)
Patrick Leahy - March 31, 1940 (age 78 years)
Nancy Pelosi - March 26, 1940 (age 78 years)
Diane Feinstein - June 22, 1933 (age 85 years)
I suppose we can consider the children:
Eric Holder - January 21, 1951 (age 67 years)
Elizabeth Warren - June 22, 1949 (age 69 years)