I've spent a few hours looking for sources that give an exact year, but only found this website putting the year to 1935. Now, I've read and heard from other historians that the British Royal Navy used bombs/depth charges to destroy the wreck during World War II. Don't really know the exact reason why, but I heard it had to do with them wanting to cover up the fact that there were weapons on board. But either way, it was likely destroyed before any professional dives and salvation efforts were made.

I don't have a source for this, because I cannot remember where I read it, but captain William T. Turner supposedly remembered the exact speed of the ship when it was torpedoed, and the path the ship continued to travel until it sank (roughly 2 miles). Having survived the sinking, this knowledge was used to pinpoint the location of the wreck, and his information was accurate enough to find the wreck a short time after World War I ended. If the use of his wisdom was truly needed, then we can safely say the wreck was found between 1918 and 1935. Compared to some other shipwrecks (especially the Titanic), this was a considerably fast discovery, though that might also be due to the shallower depth the Lusitania sits in.

Although this didn't give you a certain year (the website I linked has a year, but not much more so I personally take that with a grain of salt), I hope it gives you a rough idea of when the wreck was probably located.