Claire Patterson et al settled the Age of the Earth in 1956. In this paper, which can be obtained for free from Colorado.edu, we read:
"The following assumptions are made concerning meteorites: they were formed at the same time; they existed as isolated and closed systems; they originally contained lead of the same isotopic composition; they contain uranium which has the same isotopic composition as that of the earth." (p.1)
"If the earth is a late agglomeration without differentiation of meteoritic material then it can have any age less than meteoric material. Rather than arguing that such a process would be accompanied by chemical differentiation (and a change of the U/Pb ratio), it seems reasonable to believe instead that such a late agglomeration process would be less probable than one where both meteorites and the earth were formed at the same time. It is a fact that extreme chemical differentiation occurred during the process which led to the mechanical isolation of the mass of material of which the earth is made, and since changes in this mass were accompanied by chemical differentiation, the Pb/Pb isochron age properly refers to the time since the earth attained its present mass." (p.7)
The present age of the earth, then, is calculated upon an astronomical model which asserts that the formation of the solar system happened in the way we believe it did, which results in our assertion that measurements taken on meteorites apply to the earth as well.
Contrasted against meteorites, zircons are a nice earth-bound reference because unlike everything else in the earth's crust, we do not believe that they give falsely young readings due to the crust's constant re-melting and cooling (zircons are hard to re-melt).
If the earth really were older than the meteorites and zircons are telling us, then its long early history was probably inhospitable to life due to our observation that no zircons have been found that are older: it would have been crazy hot during that time.
It's worth noting that radiometric methods are the only methods that give an affirmative earth age greater than 0.05 million years: the ice core layers are only dated beyond that age according to curve fitting, not layer counting.