Carbon dating is good out to 25K to 50K years old. Beyond that, there's not enough C14 for an accurate test. Contamination is always a concern. Older dates often rely on datable volcanic ash or other techniques. A variety of radio-isotopes decay to lead, for example, and can date rock samples hundreds of millions to a few billion years. Zircons can go back the Earth's earliest crust 4 billion or so years ago.

In other words, there is a wide array to methods to date objects based on radioactive decay. There are also many other techniques relying on various forms of induced luminescence, geomagnetism, and of course genetic clocks. Early attempts to find the age of Earth estimated how long it took the planet to cool and how long it took oceans to reach today's salinity. These contained serious errors (we didn't yet know about heat from radioactive decay or the geology of salt deposits) but show that there are many ways to estimate age.