In India, it ends up being the availability of records that are (a) decipherable and (b) recognizably in that language. Among the so-called Indian "regional" langauges, the oldest Tamil records date back to about 2300 BC, while the oldest available records of Kannada/Telugu are a few hundreds of years younger. It's quite likely these languages existed long before these written records, but records are what decide the oldness olympics.