No idea who the fog oldman is, but there are only a handful of Shinsengumi who reached old age.
I think it might be Saito Hajime. Youngest in Kondo's group, really gifted in swordsmanship, practiced a form of swordsmanship known as "Outer Nothingness" ([無外真伝剣法] or "Mugai Shinden Kenpō"), born 1844 and lived until 1915 (71 years old) so definitely in the oldest category of known Shinsengumi, and he's fairly famous on his own.
On the other hand, it could also be Nagakura Shinpachi, who was born 1839 and lived until 1915 (76 years old) as well (died a bit before Saito in the year, in fact); he famously left the Shinsengumi a year or so before it was annihilated with a few others, standing up against Kondo and Hijikata who had wanted them to become their retainers.