There's not a simple answer for this but I can give you this as an answer: in the United States something becomes historic when it's listed on the National Register of Historic Places. Every state had their own process for listing properties. In mine, a property must be determined eligible by the State Historic Preservation Office, a nomination must be written explaining the significance of the property as it relates to a historical context, a historical figure, a architect or architectural movement, or something archaeological. Then it is reviewed by the State Historic Preservation Office, reviewed by a state advisory board, and finally reviewed by the Keeper of the National Register of Historic Places. When everyone has agreed its historic it gets listed on the National Register and is officially historic.

Normally properties must be at least 50 years old and have some sort of historical importance. Many historic districts are listed as examples of urban develoment in the particular city or region they are in at a particular time. We had someone want to list a WPA constructed outhouse. After a bit of research we found that the WPA did a lot to improve rual santitation in the 1939's and the vault of their privies had an important design. Thus if the vault was intact then the outhouse could in fact be eligible for listing. It turned out the outhouse had been moved so the vault was gone plus the exterior had lost a of its original wood so was determined to be ineligible.

As for your actual examples, arrowheads, at least where I live, are far more common than switchblades so I would say the switchblade has a better chance of bring deemed historic depending on how old it is and who manufactured it. Neither would listed on the National Register but a museum may be interested in them.