When did people first start drinking skim milk?

Because skim milk is merely regular milk with the cream "skimmed" off, skim milk has been drunk probably since regular milk has been drunk.

http://simple.wikipedia.org/wiki/Skimmed_milk

Etymology: skim
c.1420 (skimmer, the utensil, is attested from c.1392), "to clear (a liquid) from matter floating on the surface," from O.Fr. escumer "remove scum," from escume (Fr. écume) "scum," from a Gmc. source (cf. O.H.G. scum "scum," Ger. Schaum; see scum). Hence, skim milk (1596), from which the cream has been skimmed.

http://www.etymonline.com/index.php?search=skim+milk&searchmode=none

Skim Milk First Used:Henry IV, Part I, Act II, Scene iii, Hotspur Soliloquy -- “O, I could divide myself and go to buffets, for moving such a dish of skim milk with so honourable an action!”