St. Christopher metals, as well as other religious metals have been around since recorded history. Their appearance has changed very little over the years, so it's difficult to determine the age.
St. Christopher medallions were and are common for folks to have in their car (where they need protection from accidents). Often they would have a clip to hold them to the sun visor. I'd guess it had been silver or nickle plated (which sticks well to the copper that remains).
Looks like it may have had a clip on the back of it. So if it did clip on a cars sun visor then that would give me an indication of its age.
Don't see any plating but I'm sure it was at one time.
The legend of St. Christopher was that he was a very tall man who lived near a river with rapid currents and was often called upon to help people across. One day the Christ child appeared to him and he carried him across the water on his shoulders. He was martyred for refusing to deny his faith. He is the patron St. of Travelers.
My Italian grandmother always kept a St. Christopher medal in her car, pinned to the visor and bought these medals for her children and grandchildren when they got cars of their own.