The observations sheet describes the inclusive age span a count
was drawn over (age_min..age_max), but the overwhelmingly common case is
a count at one age. Rather than make every such row repeat itself, a sheet
that carries a lone age column is read as age_min = age_max = age.
Only applied when neither span column is present: a sheet that supplies
age_min/age_max is authoritative, and any age column alongside them is
an ordinary ignored extra column.