Searching more information about Steinthal Estates, I stumpled upon DarjeelingNews.net, and their history of the region's tea: http://www.darjeelingnews.net/darjeeling_tea.html
And I quote:
The first commercial tea gardens were Tukvar, Steinthal and Aloobari tea estates. This was in 1852 and all these plantations used seeds that were raised in the government nurseries. By 1856 the experimental stage had been passed and development was rapid. According to Darjeeling Gazetteer, Alubari Tea Garden was opened by the Kurseong and Darjeeling Tea Company and another on the Lebong spur by the Darjeeling Land Mortgage Bank. Several hundred ha of forest land was cleared, from 750 m elevation above the sea to 1800 m. By 1857 25 or 30 ha was planted , besides six nurseries, in which a ton of seed has been sown during 1857.So the Steinthal tea estate is one of the first three that were established in Darjeeling, and some of their tea trees date back to that period.