Which primary tooth is generally accepted as the first to erupt, and at about what mean age?

Eruption for the primary mandibular central incisor may be given as 6 months (Woelfel and Scheid, 2002); 6.5 months (Kraus et al., 1969); 7.5 months(Charlick et al.); or 8 (6 to 10) months (Ash and Nelson, 2003).