The archaeological evidence for use of salt by humans is old, and definitely prehistoric (ie before the age of written evidence). The (to my knowledge) oldest known site of salt mining is in Hallstatt, Austria, where archaeological evidence found proof of salt being mined there before the first millenium BCE, but it is reasonable to assume that the salt-rich mountains there were the target of anyone who settled there (as it offers little other resources or agricultural riches and is a hard area to live in in winter especially in prehistoric times).The oldest archaeological find at Hallstatt (a shoe) dates to ca. 5500 BC, and it is thoroughly reasonable to assume that salt was mined there even then, which would make salt mining at Hallstatt an over 7,000 years old practice. The most famous archaeological case there is the legendary "man in salt", who was found inside the mountain in the mid 18th century when a mining shaft was dug and uncovered an older mining shaft which had collapsed in the past and crushed someone. He was pulled out and immediately reburied according to Christian custom. This man had been described thoroughly in the local town priests diaries, and was (wrongly) assumed to be an early Christian. Later archaeological finds of burial corpses from the Bronze age fit exactly the description found in the town chronicle of the "man in salt"; which leads us to reasonably assume that even in 1200 BC there were already Bronze age deep salt mining operations going on. Furthermore a ladder found nearby was carbon dated to be 3,000 years old (and what do you need a big ladder for in the area if not for salt mines).

Salt, even in prehistoric times was primarily used for conserving food, and not necessarily needed just for taste: with salt, you can preserve food to make it through winter, and its value in harsh winter areas is therefore much higher than in areas with mild moderate winters. Salt meant survival -- and was therefore a symbol for life in many "primitive" cultures. (As an interesting footnote, it should be added here that, to run a successful salt mining operation, you actually need a fairly developed society, as it requires various separate full time experts with specialization in their fields -- toolmakers, miners, loggers etc -- IIRC you need at least 7 separate specializations for salt mining alone, never even mind bronze making, which also requires extended trading networks on top; so you need a society of deep cooperation, where some provide food for those who can make tools etc. "Primitive" is something else).

Maybe someone with the knowledge of history of cuisine knows where the first mentioning of salt occurs in a recipe, but my hunch is that it's older than written recipes themselves.