Persian is an Iranian language belonging to the Indo-Iranian branch of the Indo-European family of languages. In general, Iranian languages are known from three periods, usually referred to as Old, Middle, and New (Modern) periods. These correspond to three eras in Iranian history; Old era being the period from sometime before Achaemenids, the Achaemenid era and sometime after Achaemenids (that is to 400-300 BC), Middle era being the next period most officially Sassanid era and sometime in post-Sassanid era, and the New era being the period afterwards down to present day.
According to available documents, the Persian language is “the only Iranian language” for which close philological relationships between all of its three stages are established and so that Old, Middle, and New Persian represent one and the same language of Persian, that is New Persian is a direct descendent of Middle and Old Persian.
The oldest records in Old Persian date back to the Persian Empire of the 6th century BC.