They don't seem to be a scheduled service operator. They look like a charter airline.

Common definition of a "commercial operation" for airlines is scheduled operation, not charters.

So... That boeing just may be the oldest flying airliner on scheduled routes.

It appears you are correct in this regard and the websites I used are not.

Are there any such planes used for regular scheduled (passenger) airline services? It appears to me that Buffalo Airways is charter and cargo only.

They have two Curtiss C-46 from 1945. Its interesting that its cheaper to continue to pay for the maintenance and fuel than lease something more modern.

Replacements don’t exist. Those old prop liners can operate off unimproved dirt strips, grass, even snow and ice - and off short runways too.

Jets don’t like that.

For back country missions the only thing maybe better than a DC-3 is a Basler... and that is a modernized DC-3 with reinforcements and turboprops for more power. They suck a lot more fuel than a plain DC3 though, which can be problematic as some of these fields they have to fly fuel in.

A C130 should be a viable modern alternative for those use cases, if economic

A fully loaded DC-3 can takeoff in just over 2000ft. A C-130 needs almost 4000ft.

There are no small planes that can replace DC-3. C-130 is massive!