Carbon 14 dating used to be reliable back to about 35,000-40,000 years, but advances in shielding and now AMS dating have extended it to about 50,000 years. They are also working on advances which may make 80,000 years possible, but that is still a few years away most likely.
This is one of my fields, so let me know if you have any other questions.
C-14 is only accurate to about 50,000 years. The decays of other elements are used to determine older dates. Potassium-argon dating for example, is reliable back to hundreds of millions of years; Uranium and lead trapped in zircon crystals can give dates going back billions of years. Elements that are used to date more ancient ages are conversely not accurate for more recent dating, which is why Carbon-14 is relied on to date relatively recent prehistoric artifacts.