Radiocarbon dating of plant seeds in the same soil is 21,000 to 23,000 years ago.
The fossil and DNA record of humans in North America is quite rare between 23,000 and 8,000 years ago.
One theory - most settlers lived on the Pacific Coast, which flooded away when glaciers began to melt.
End of the Clovis Age was 12,500 years, more or less.