It’s the same as chrisomes. Infant mortality was so high, the ones who died without obvious cause just get lumped together by age group.

Chrisomes refers to those who died within the first month, during the time they’d be baptised. The baptismal cloth, the chrisome, would often be just as a burial shroud.

Teeth meant they were old enough to have one or more teeth, 6-24 months. Teething was thought to be potentially fatal because so many infants died during that period. Correlation, causation, yadda yadda yadda.