Keeping in mind that the core group initially formed in 1960, and that Chad Mitchell left his own group in 1965 -- which would seem to signal the end is nigh, especially for a folk-singing trio as the folk music era was nearing an end -- and that the group reformed as the Mitchell Trio after its leader left and that this core three reunited in 1987 for a TV special...it's remarkable that they continued performing together at that point, and here's a video of them on stage only nine years ago! In 2010.
In fairness, they didn't performed regularly together during all those year, but it's still seriously impressive. Especially since they remained in wonderful voice.