1.32cm = 0.52" . . . but also 13.2mm?? A little small for a .54 caliber rifle but if it is 12.7mm that is pretty much a 0.50" ball. Which is a VERY tight it in a .50 cal rifle (you'd need a metal rod and wood mallet to load).
As rifles got tired it was common to freshen the bore and use a slightly larger ball - which may be the case with yours. You really need a micrometer (thousands of an inch, hundredths of a mm) to be sure with bullets.
Age? Sometime after 1400 to present. I still hunt deer with cast round lead balls and a flintlock for the sport of it. They have been in continual use in some parts of the country since flint tipped arrows fell out of favor.