wiki entry for sanskrit says it is as old as 3700 year old. Now regarding script, I know that none of south indian Brahmins used Devanagiri script until very recently.It was Granthavari or Granthakshari which was used. even present Malayalam script is used to write sanskrit. Vedas were taught orally. memorising is imortant. None of Vedic acharyas referred books. To them whole Vedas, Yajur,rg and sama ,samhitas, sakha, and padams were byheart. They first teach pada Padha(Words i vedas; one by one) and then Vedic hymns. Sanskrit was also taught this way. Sankskrit scholars were masters in Kalidasa's works like Raghuvamsam, sakuntalam. They will start with a sloka, teach all individual words, its' meanings, sandhis(combinations of words),samasams, Naanarthas (same word with different meanings with diff context; or words which are pronounced similarly with slight spelling differences) ,paryayas(many words with same meaning), samasams etc etc. Now the disciple also learn the alphabet, the a aa, i, ii, u,uu and the consonents like ka,kha,ga,gha nga etc and he knows this bcoz Malayalam too have the same alphabets. He never learn to write. It is not taught using books, slate and pencil. Todays peoples cannot imagine learning a language without using books and writing. So in short in the past, Brahmins,Kshatriya and Vysya spoke sanskrit, sudras spoke bot and outcastes regional languages like Tamil,Malayalam, Kannada, Magadhi,Pali,sauraseni ,Gandhari, etc etc...
