You can learn improvisation as soon as you learn how to properly play the violin/fiddle. The approach of learning it can be very unique though depending on your music background. If you have been playing violin for a long time, you will want to take a very structured music theory approach to it. If you have played more fiddle (and not violin), you will want to take a more musical ear approach to it. It is actually difficult to improv when you are very used to classical playing so really the earlier you start the better. Other techniques including sliding, grace notes, drones etc. are actually helped by classical playing. Having good left hand technique helps all of these techniques I just mentioned. I would say slides comes first, grace notes second, and drones thirds. Improvisation could come anywhere in there!