The answer to your question is complex. I'm going to open with an excerpt from my recent book, The Folklore of Cornwall: The Oral Tradition of a Celtic Nation, because it answers part of what you're after:

There is evidence that people have always thought their beliefs in the supernatural were fading and that earlier generations were more fervent in their fairy faith. Asserting that a belief in these entities was a bygone facet of English heritage features in Chaucer’s fourteenth-century introduction to ‘The Wife of Bath’s Tale’, which the character sets ‘In the olden days of King Arthur [when] … all this land was filled with faerie’. The Wife of Bath adds, ‘This was the old belief’. It is a theme that appears to have resonated over the centuries with a repeated assertion that people regarded those from previous centuries to have possessed a stronger faith in the existence of a fairy world. Writing in 1997, Linda-May Ballard cites Jeremiah Curtin as describing the idea of a waning belief in the fairies in his 1895 publication on Irish folklore. Ballard then poses the question, ‘Might it be that the idea that fairy belief is fading and belongs to the past, is part’ of the wider tradition embracing the belief in these supernatural beings?' (sources: John H. Fisher, editor, The Complete Poetry and Prose of Geoffrey Chaucer (New York: Holt, Rinehardt and Winston, 1977), p. 120; Linda-May Ballard, ‘Fairies and the Supernatural on Reachrai’, in Narváez, The Good People, p. 91; note 9)

... Seeing Fairies: From the Lost Archives of the Fairy Investigation Society, Authentic Reports of Fairies in Modern Times, provides evidence of British tradition enduring into at least the mid-twentieth century. Modernism affected but did not extinguish fairy traditions. A Cornish example from 2017 reinforces the idea that while folklore may change, aspects of belief can defy intuition by lingering over time. The Packet, a newspaper serving Falmouth and Penryn in Cornwall, reported the one-hundredth birthday of Falmouth native Molly Tidmarsh. The centenarian implied that some of her good fortune in living so long may have been due to her birth under a ‘piskie ball’, a round lump of clay, fired together with one of the tiles used on the roof ridgeline of her family’s home and business. Molly suggested that these objects were created to distract piskies who sought to come down the chimney to cause mischief for the occupants of the house. Instead, the piskie ball would entrance them, and they would dance around it until dawn, at which point they would disappear. It is unclear, and largely unimportant, if Molly Tidmarsh believed good luck was hers because she was born under the ball; what matters here is that piskies featured in a newspaper article in 2017 without a need to explain what they were. Molly remembered a tradition of the early twentieth century and it still resonated with readers one hundred years later. (sources: Marjorie T. Johnson, Seeing Fairies: From the Lost Archives of the Fairy Investigation Society, Authentic Reports of Fairies in Modern Times (San Antonio, Texas: Anomalist Books, 2014); The Packet, 22 August 2017.

In other words, the impression that fairy beliefs in Northern Europe were stronger in previous generations than they were during the current one (whenever that happened to be) is long-standing. In addition, the idea that fairy beliefs are now dead is often asserted, and yet it is simply not true.

That said, modernism, industrialism, and increasing urbanism has curtailed the fairy faith - even in Europe. I spoke with a young student at university in Dublin, Ireland in 1982; she was from a rural place in the west of Ireland. When I asked if she believed in the fairies, she said that she did when she went home, but she didn't when she was in Dublin. That was an eloquent description of the issue that urbanization causes/caused with regard to this element of folklore. Despite all of that, there are still people who are believers as part of an inherited tradition, and there is still a tendency to see belief as in a steady state of decline - just as it has been viewed for decades if not centuries.

That is the case that can be made for belief lingering. That said, there is no question that in an urban setting in particular, belief in fairies has tended to die out, remaining exclusively as something that children believe in. For urban dwellers, that process was well underway by the mid twentieth century, and we can see the roots of that process earlier - but it was a process, different from one place to the next and one family to the next, so it is not possible to speak definitively about precisely "when ... the belief in fairies" underwent this transformation.

In North America, the problem is a bit simpler - but even here there are complications. In general, European folklore tends to survive only with first generation immigrants. We can't speak of a specific time when the belief disappeared because, again, individuals passed through the first generation phase at different times, but this was the tendency. There are, however, notable exceptions. Nova Scotia has a well-documented ongoing, fairy tradition transplanted from Scotland. I had a student in 1980 who lived in a rural village in Alaska, settled by Scottish and Welsh miners - and the fairy faith was thriving there when she took my class.

I have also spent a great deal of time studying the transplanting of the Cornish knocker, the underground mining fairy that became the tommyknocker in the American West. This tradition not only survived, but it became part of miners' folklore regardless of the origin of each miner. In 2007, I received an account from a miner in Golconda, Nevada who had an encounter with tommyknockers in the 1950s. When I asked if he were of Cornish ancestry, he explained that he was Portuguese - and the tommyknockers he encounters were as real to him as if his ancestors had come from Cornwall. This tradition is largely dead now, but it survived for a very long time, so again, giving a date for the extinction is impossible.

I have published extensively on knockers/tommyknockers - a subject that I treat in my book, The Folklore of Cornwall (which is too expensive). Free access is available to articles here - with a couple of fun illustrations - and here - which is my original study, now a quarter century old and superseded to a certain extent by subsequent research and consideration.