A total of 799,000 babies were born in 2015, according to the study published by statistics agency Insee on Thursday, representing a decline of 2.4 percent compared to the previous year.
Just over five percent of these mothers were aged over 40, compared to four percent ten years ago, and only one percent in 1980.
The proportion of "late births", those where the mother is aged 40 or older, is similar to figures in the postwar period, however Insee noted a shift in the reason for late births.
In the 1950's, the number of over-40-year-olds giving birth was due to the "frequency of large families", explained Isabelle Robert-Bobée, Head of Demographic Studies at the agency.
The proportion of late births then fell following the invention of the pill and the decline in the number of large families, and has picked up again as the average age at which women have their first child has got later and later.
One reason for the increase in older mothers is that more French women are choosing to pursue their career in their 20s and 30s, Robert-Bobée added, and have children later in life. Over a quarter (26 percent) of the women over 40 who gave birth were first-time mothers, a significant increase from 17 percent in 1981.
Compared to other European countries, France has a high fertility rate, and a slightly lower proportion of "late births". In Italy and Spain, where birthrates are lower, the proportion of late births in over nine percent.
The average age of mothers in France was 30.3 years, compared to 31.5 in Italy (where the government recently launched a widely criticized campaign to encourage having children) and 31.8 in Spain.
Dads, however, are likely to be slightly older; 136,000 babies were born to men aged over 40 (16.9 percent of all births) - while two percent of dads had already celebrated their 50th birthday.