When it becomes *known* folk music is whenever the term was created and first used in the context of labelling a specific type of music - so that is mid-19th century, according to others above.

Why it becomes known as folk music - what puts a song in that `folk' context - is the logical next step, but a step fraught with subjectivity. For example, one possible definition of the folk context is `if the folk sing it because they enjoy it rather than have to'. For example, someone may think The Holly and The Ivy is a good song in its own right, but its contexct as a Christmas Carol means it has a place and time which defines when it can be sung.

On the other hand, many of the songs of Buddy Holly or the Everly Brothers have obvious roots as pop songs. But if someone sings one in a crowded place - a pub is the obvious place - people of all ages join in, at least with the choruses. They have become part of the collective psyche that give pleasure, fond memories (and perhaps not so fond but nostalgic memories). And people like singing together, they like to participate. They'll say they don't sing and don't know any songs but they are right there with the chorus of Bye Bye Love or a shanty like South Australia.