John Hawks Anthropology Weblog : Longer life with younger mothers:Paternal age had no affet in this study.
...They also discovered that being the first-born in a family meant a lot, boosting the odds of making it to 100 by nearly 80 percent...
... statistical analysis revealed that young maternal age at birth completely accounted for the first-born effect.
Everyone's first guess is that it's the age of the mitochondria that matter, in humans virtually all mitochondria are maternal. One wonders how many "first born" effects are really effects of maternal age.