As readers of Guénon well know, the concept of the kali yuga or dark age is central to his thought, and thus to Traditionalism as a whole. An excellent new article on the Hindu origins and European uses of the concept places its development in a wider context. The article is Luis González-Reimann, "The Yugas: Their Importance in India and their Use by Western Intellectuals and Esoteric and New Age Writers," Religion Compass 8 (2014), pp. 357–370.
González-Reimann argues that the concept of the kali yuga is a late one, emerging in India only around the first century AD, when it helped to explain the various disasters then afflicting the classical Vedic system. It became known in Europe during the seventeenth century, but did not attract much attention until the eighteenth century, when Voltaire was among those interested in it, and in the challenge that the system of the yugas presented to established Christian chronology. The intellectual mainstream soon lost interest, however, according to González-Reimann because of the impact of a refutation by Sir William Jones. In fact, I suspect, it was also because chronologies based on geology were then beginning to render all other varieties of chronology obsolete.
Even if the intellectual mainstream lost interest in the yugas, esotericists did not. The yugas featured in the work of Antoine Fabre d'Olivet (1767-1825), who suggested that the Hindus had got them the wrong way round and that the kali yuga was actually the best of the four yugas. They then appear in the work of Alexandre Saint-Yves d'Alveydre (1842–1909), who agreed with Fabre d'Olivet that the yugas were actually improving. Helena Blavatsky, Annie Besant and Rudolf Steiner all wrote about the yugas, following Saint-Yves d'Alveydre's optimistic view. Besant emphasized the coming new age of the satya yuga, and Steiner even held that the kali yuga ended in 1899.
Saint-Yves d'Alveydre modified the standard Indian understanding of the yugas, equating the mahāyuga with the manvantara, two measures of time that classically belonged to different systems, and were far from identical. Guénon follows Saint-Yves d'Alveydre in this, which indicates his proximate source. Guénon differed from the esoteric norm, however, and most importantly, in reverting to the original emphasis on the negative nature of the kali yuga itself. And the rest, as they say, is history. Once again, we see how Traditionalism owes much to mainstream esotericism, but also differs from it.
All this is shown clearly by González-Reimann in his article, which closes by observing that "in the second half of the 20th century, esotericism largely morphed into New
Age thinking, or ... the New Age engulfed esotericism. Either way, such ideas [as the yugas] ... have been incorporated into the manifold spectrum of New Age
thought." Yes, perhaps, so far as optimistic understandings from Fabre d'Olivet and Saint-Yves d'Alveydre to Besant and Steiner are concerned, but no so far as the Traditionalist understanding is concerned.