As with most things in history, it's difficult to point to an exact moment that things changed. At varies times throughout (Western) history, actors and singers were welcomed into high society, but more as pets than as equals. (I am focusing more on British theatre history here than anything else as I have a deeper knowledge of it, but there are certainly some parallels in other European cultures.)

Shakespeare, for example, was regularly invited to court and performed for nobility, but the closest he came to class mobility required using money made as a theatre shareholder (not playwright or actor) to convince the heraldric college that his father John had obscure noble connections via Henry VII. In 1602, just six years after the Shakespeare Coat of Arms granted the Shakespeare arms (and a year after the arms passed from John Shakespeare to William, there was a minor scandal in the college when one of its members raised a fuss that arms had been granted to a number of "base persons," including "Shakesepeare ye player." Being a theatre owner gave Shakespeare the cash to buy his way in, but being an actor meant he was always kept outside.

Nell Gwynn, one of the first actresses of the English stage after the Restoration made it more legal for her to be there, certainly moved in royal circles - as the mistress of King Charles II. Not much open respect to be granted there in a class conscious society. David Garrick in the 18th Century was beloved by wealthy theatergoers, but any social cache he had came from his status as a theatre manager. He, like Shakespeare before him, could only claim class status as businessmen. That could buy you a kind of respect like that afforded a wealthy merchant, but the doors of nobility stay closed.

The real turning point - or rather a road sign that the turn had already happened - has to be the actor Henry Irving receiving a knighthood in 1895. It was the first time anyone was afforded noble status not in spite of being an artist, but because of it. As for why this change eventually happened, there are a lot of available arguments. Personally, I'd suggest that the peak of Victorian Empire required as much cultural exportation as economic. Throughout the Nineteenth Century you can find more and more cases of actors held up as national ideals - Clairon and later Bernhardt in France, Booth and Forrest in the US, Macready and Irving in the UK. This holding up of actors as national(list) ideals was the biggest cause of the Astor Place Riots of 1849, when the aforementioned Forrest and Macready were schedule to play *Macbeth* on the same night, stirring up all sorts of nativist vs. immigrant sentiment. The resulting riots led to roughly thirty deaths.

A figure like Irving, who could claim a new type of realism and honesty in acting (though most generations make the same claim) white maintaining a profoundly conservative and patrician air, was an excellent archetype for the nobility. So much so that his manager, Bram Stoker, based the physiognomy of another aging patrician on Irving - Count Vlad Dracula. Knighthoods for actors became more common over the years, and the transformation was entirely complete when Laurence Olivier was awarded a life peerage and a seat in the House of Lords in 1970, making him an official part of the upper gentry. You could make a case that Meghan Markle being welcomed into the Royal family represents yet another new step as well.