https://www.nature.com/articles/38148 [study from 1997]

Theories on why this is:

• The hormone progesterone might slow aging in women. It's produced much more during and after pregnancy. Which makes sense biologically: mothers need to stay fit as long as possible to care for their offspring
• Having kids in their 40s just skews the data to healthier individuals. Those that can still have kids in their 40s are healthier. And healthier people live longer
• It's just money. Successfully having kids in their 40s is a thing for higher socioeconomic class. And richer people can better afford the healthcare to live longer at the end of their lives

No matter the cause: Fertility is a remarkable longevity biomarker.