Not sure when, but in the beginning of photography, people did not smile, because exposing the plate took several minutes, and forcing yourself to smile for that long was not feasible.
I suspect people started to smile, when exposure became fast enough.

It dates back to at least the 1940s, when people began to “Say Cheese”. But before that the spread of movies n the 1920s encouraged everyone to smile like they were a movie star.

The first photographs were formal pictures taken in a studio and people didn’t smile. I’m guessing smiling began when Box Brownie cameras were introduced to the masses around 1900.

I would say smiling became the norm in American casual photography in the 1920s, though the bases were in place much earlier.

George Eastman introduced the disposable camera in 1988, which had a capacity of 100 frames. You sent the camera back to Kodak for processing. $10 netted you prints and a new camera. $10 was still expensive, though. It took decades for the cost to make photos throw-aways.

Here’s a neat photo-stream of smiling folks from the days when it was not common – The Smiling Victorian

“George Eastman introduced the disposable camera in 1988”

1888, of course.