One way to look at the years would be that there was an industry of sheet music of partly or wholly 12-bar "Blues" songs that only started up in 1912. So if someone turned 18 or older in 1912, which all those people did, and they were Southerners, which they were, they'd generally had a chance to hear folk blues before 1912. As Sam McGee e.g. recalled he did. Pete Steele was about 21 in 1912, Tom Darby was about 22, Bert Bilbro was about 23, etc.