[The use of the age concept in monthly population figures]

"In Dutch population statistics age is usually defined as the year in which a demographic event occurs minus the year of birth. This age concept is referred to as age on 31 December. As a consequence of determining age in this manner, interpreting age-specific monthly figures is sometimes difficult.... Statistics Netherlands prefers the concept of age on 31 December for two reasons: it facilitates the linking of cohort and period data in the case of annual figures; using one concept for annual and monthly figures avoids consistency problems." (SUMMARY IN ENG)