Here's a real mess which defies easy logical analysis. Governments around the world are asserting that they own anything which is defined as an "antiquity."

And worse yet, the US Court of Appeals for the Second Circuit held that,

"foreign national ownership laws can make antiquities "stolen" under the US National Stolen Property Act (NSPA)"

In other words, the passage of time creates a sort of confiscation of property which traditionally would have belonged to a landowner or a finder, and expropriates it in the name of whatever government passes the law.

Doubtless, governments would reply that this is in the interests of the public good. That antiquities are the common property of all the people living in a country, and it is not fair to allow any individual to own them. What is the functional difference in logic between that and Stalin's, Mao's or the Khmer Rouge's confiscation of all private property as common property of all? Simply the age of the property? Its value? Oil, coal, and precious metals in the ground are also very old -- and they are finite in nature. Is it fair to allow a property owner to own these things? Shouldn't they be the property of everyone?

Or is the cultural value of the object to be the determing factor in its expropriation? Why wouldn't any great work of art regardless of age be properly subject to government confiscation as part of a "cultural heritage?"

And, in the case of antiquities, suppose we trace the property back to the people who created it. Governments today can hardly be said to stand in the shoes of various occupying powers which may have created or produced the antiquities, or once owned the land. If you dig in your yard in the United States, you might find coins from the British, French, or Spanish Empires. Assuming that these countries passed appropriate laws, why wouldn't you now be a "thief" for selling them, or for buying them from someone who dug them up?

As to definitions, I do not think that there is any standard rule. For purpose of example, here is Israel's definition:

An antiquity, as defined by this law, refers to (1) an artifact produced before the year 1700 CE.; (2) a human-made object of historical value made after the year 1700 CE., and declared by the Minister of Education and Culture to be an antiquity; or (3) a biological fragment dating from before the year 1300 CE.

It's no longer finders-keepers!

(Such laws would, I believe, be unconstitutional in the United States if passed here, because of the prohibition on taking private property for public use. But who am I to say that? Since when has the Constitution meant anything other than what the courts say it means?)

ADDENDUM: The Fifth Amendment to the U.S. Constitution provides that no "private property be taken for public use, without just compensation." If the government had wanted to expropriate, say, the Golden Plates, finder Joseph Smith would have been entitled to compensation. (I don't know the legal status of the angel Gabriel, who may have been receiving stolen property -- either Indian or antiquity.)