Lower wisdom teeth – #17 and #32
The last teeth at the very back of the lower jaw.
Roots
Usually two, but wisdom teeth vary a lot: in a Korean scan study, 57% had two roots and 38% had one.
Canals
Most often three (64% in a Central Indian scan study). We haven't found a reliable range beyond that.
These are typical counts from scan studies. Teeth vary, and only an X-ray or scan shows what a particular tooth has.
When they come in
Usually at 17–21 years old, according to the ADA's eruption chart.
No baby teeth come before them: adult molars come in behind the baby teeth.
They bite against the upper wisdom teeth.
Sources
Park et al. 2013; Somasundaram et al. 2017; American Dental Association 2012; Harris 2005. Full titles are on the About page.