Tolkien was born a Victorian, and as a Germanic scholar and philologist, he wouldn't have been out of place had he spent most of his adult life in the nineteenth century instead of the twentieth. And observe the energy with which the Prussians studied the Teutonic knights and with which the Victorians studied Gothic architecture and mediæval history; I think the cast of mind of people was receptive enough, and Robert Jordan could have published door-stopping cycles of fantasy to widespread readership in 1890, had he been around, and had the publisher to do it.
I don't have any reason to doubt that the Grimms' _Kinder- und Hausmärchen_ were descriptive of the extant folklore, and that folklore was pretty dark and un-twee--not really children's stuff. But it didn't involve swords or knights; the field needed a Tolkien to spark it off into its current shape, and the guy was a one-off.
Jules Verne invented modern science fiction, with both an adult and child readership, and since him, it hasn't, at all, been purely kids' stuff--Brave New World sparked debate and is a standard cultural reference point, Arthur C. Clarke and Issac Asimov are hardly obscure and puerile. I'm not sure it's even perceived as childish; what child would be interested in Kubrick's version of 2001?
And, imagine there had been two networked computers in your house and networked Quake--for which the storyline was pure fantasy--was available. Do you think he would have had any qualms about joining in? I doubt it.
Fashions change, and vary. It's almost impossible to say why. A hundred years ago, playing cards was as pervasive a hobby as watching football is now (okay, perhaps that can be explained by television; but what about in pubs? It's as effective a way to pass time when drinking now as it was then.) I believe playing chess is a pervasive habit in Russia, down to the smallest hamlets; you'd be looked at like you had two heads if you suggested it as an appropriate way to spend an evening in male company down t'country here.