Which language is oldest in India?
Some of the oldest known languages in the Indian Subcontinent were Pali, Sanskrit, Brahmi, Prakrit. Prakrit definitely predates Sanskrit, but Sanskrit was popularized as a "marga" or "higher" language by the monarchs at a certain point after the "Aryan" influence. Several scripts too were known, such as Kharoshti, derived from Aramaic. The Indus Valley "script" has not been deciphered. None of the older languages are in use today.
Regional languages as we know them today were later developments- some well into the 11th century CE. These languages were derived from both the Sanskrit as well as Dravidic roots. Tamil, Kannada, Telugu show greater Dravidic roots than say Hindi, Bengali, or Oriya, which show greater Sanskritic influence.
Sanskrit, however, is still studied by many for academic and religious reasons. By inference, smaller groups of specialists also study the other archaic languages. But these are not in use even to the extent that Sanskrit is.
The division of states on the basis of language is a post-independance phenomenon, and has been cobbled together from many kingdoms, princely states, British Presidencies and other non-linguistic political divisions.
While it has been criticized by many as "divisive", others have hailed it as a successful experiment.