* Proto-Indo-European (PIE): The ancestor of all Indo-European languages is a reconstructed language known as Proto-Indo-European. We don't have any written records of PIE, and its existence is inferred from studying the similarities between its descendants.
* Reconstruction is not the same as the real thing: We can't directly compare the age of PIE to the age of existing languages. The earliest attested Indo-European languages are from around 3,000 years ago, but PIE is believed to have been spoken around 4,500 to 5,000 years ago.
* No single "oldest" language: Languages evolve and change over time. While some languages might be more conservative (preserving older features) than others, they are all continuously developing.
Instead of looking for the "oldest," it's more helpful to think in terms of:
* Earliest attested: The oldest documented Indo-European languages include:
* Hittite (c. 1600 BCE)
* Mycenaean Greek (c. 1450 BCE)
* Vedic Sanskrit (c. 1500 BCE)
* Most conservative: Languages like Avestan (an ancient Iranian language) or some dialects of Lithuanian preserve many features thought to be closer to PIE.
In summary: While we can't say definitively which is the oldest, we can identify the earliest attested Indo-European languages and those that retain features closest to Proto-Indo-European.