I can not be certain of the exact age, but when I picked it up I guessed that it was from the 1970's or 1980's. After a little bit of online research, I found a forum posting that showed a duplicate of this buckle. The poster stated that he received the buckle and a department patch, from a Mohave County Deputy in 1979. It looks like my guess was right!
The buckle appears to be worn a bit smooth on the surface, which indicates that it was probably worn on the duty belt, behind the Sam Browne gun belt. The buckle is unmarked so there is no way to tell who did the casting.
In the 1970's, Mohave County, Arizona, was a very rural "Wild West" county with a very low population of around 50,000 or so. Today the population has grown to over 200,00 and the sheriff's department is a very modern, 21st century department.
Mohave County Sheriff's Department had it's moment in the "Hollywood Sun" back in 1959, when it was featured prominently in the movie "Edge of Eternity". The movie featured a Deputy who solves a grisly, multi-victim murder at the edge of the Grand Canyon. The highlight of the entire film is when it ends with a dramatic fight scene in a cable gondola car above the canyon!
Here's a New York Times review of the movie from 1960:
Here's a shot from the movie showing one of the Mohave County Sheriff's Department patrol cars and a deputy. The car on the left is from the Arizona State Patrol and the car on the right is from the Sheriff's Department.
And here is a movie still that shows the old Mohave County uniforms. The shot is from 1959, but I would imagine it would still be the same in the 1970's.
And to finish things up, here are a couple more shots of the buckle.