Burgan knew John Stewart as a friend at least as far back as their high school days at Damien in Pomona in the late 1950s, so I'll hazard the guess that Jerry was around 80 years old, maybe just a bit younger than John.
We Five charted only 3 singles and 2 albums in their history, but thanks to Burgan and his wife Debbie (who replaced Bivens as the group's female lead vocalist in the late 1960s), the group's name and repertoire has been a presence in California as long as I've lived here, most naturally around LA and in the Bay Area. One of the regular stops for the Burgans was the Coffee Gallery Backstage, near me in Altadena CA, as well as McCabe's in Santa Monica, so I saw Jerry perform at several points in all of the five decades that I've lived here. He was a solid professional, a very good singer and guitarist, but more than that an intuitive and natural entertainer.
Much more in these links. I'll miss seeing the Burgans at the Coffee Gallery when this interminable quarantine ends. Their performances were a comforting affirmation that yes, the music back then actually WAS better.
The classic lip-sync from The Hollywood Palace in 1965, re-edited skillfully for YT:
The group's third single - never charted, though I heard it on the radio in Chicago. It was written by John Stewart and sounds like it: