How Old is the Earth?
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From the US Geological Service: The results show that the meteorites, and therefore the Solar System, formed between 4.53 and 4.58 billion years ago. The best age for the Earth comes not from dating individual rocks but by considering the Earth and meteorites as part of the same evolving system in which the isotopic composition of lead, specifically the ratio of lead-207 to lead-206 changes over time owing to the decay of radioactive uranium-235 and uranium-238, respectively. Scientists have used this approach to determine the time required for the isotopes in the Earth’s oldest lead ores, of which there are only a few, to evolve from its primordial composition, as measured in uranium-free phases of iron meteorites, to its compositions at the time these lead ores separated from their mantle reservoirs. These calculations result in an age for the Earth and meteorites, and hence the Solar System, of 4.54 billion years with an uncertainty of less than 1 percent.
International Planetarium Society on the same topic:
The results consistently indicate an age of about 4.6 billion years for the Earth… These measurements of age are accepted by nearly all astronomers, including both research astronomers and planetarium educators. These astronomers come from nations and cultures around the world and from a very wide spectrum of religious beliefs.
A fundamental reason why these ancient ages are so widely accepted by the scientific community is that they are derived from several independent lines of evidence accumulated by independent and often competing teams of researchers. Each method involves different measurements and the application of different physical principles to derive ages from those measurements. The physical principles include the same thoroughly-proven principles that underlie the technology that runs the modern world. Hence the fact that the independent methods all yield similar ages reinforces confidence that the methods are sound and accurate despite their complexity and do not contain major fundamental flaws.
A second reason why these ages are so widely accepted is that for scientific results to be published in research journals, they must be critically reviewed by other scientists who are experts in the same research area. This process is called peer review and is employed in nearly all research journals in the physical and biological sciences and in the humanities and social sciences. Often the reviewers are competitors of the author and thus are especially keen to find flaws in the proposed publications. As a consequence of such review, nearly every paper must be revised and improved before it is published, and some papers are rejected because the review exposes flaws in the measurements or in their analysis and interpretation.
A third reason why these ages, and other scientific paradigms such as Einstein’s theory of relativity, are so widely accepted is that by the nature of its acquisition–through independent lines of evidence and always subject to scrutiny–scientific evidence is built up only very slowly, one step at a time. Only when a very large and diverse body of evidence has been accumulated is a broad conclusion accepted. Even then, a broad conclusion remains subject to inspection, as further evidence may reinforce or refine it, or in rare cases, overthrow it.
And here is an easy to understand version:
