But you still don't have a source for that assertion?
Risk of chromosomal abnormalities, with emphasis on live-born offspring of young mothers.
B B Little, S M Ramin, B S Cambridge, N R Schneider, D S Cohen, L M Snell, M J Harrod, and W L Johnston
In a large public urban hospital obstetrics service with > 123,000 deliveries in a 10-year period (1980-89), the frequencies (0.12%) of any type of chromosomal abnormality and of trisomy syndromes were analyzed for maternal age-related risk, by logistic regression. Focusing on very young gravidas, we found that in the study period there were 9,332 births (7.5% of all deliveries) to mothers < or = 16 years old. Estimated risks of chromosomal abnormalities among offspring associated with very young maternal age (9-16 years) were similar to those age-associated risks of mothers 20-29 years old. Risks of chromosomal abnormalities increase with advancing maternal age and are independent of ethnicity.
Still, against popular belief, most children with Down?s syndrome are born to young mothers: 51 % to mothers under 30, 72 % to women under 35.
and also the article:
Home Age and birth defects
Birth defects higher in older and younger women
With an overall prevalence of 3%-5%, babies born with birth defects are the leading cause of infant death in the United States. Down syndrome is the most common chromosomal cause for birth defects. The most common non-chromosomal birth defects are congenital heart defects, cleft lip and palate, and abdominal wall defects (gastroschisis or omphalocele). New data suggests that if a women gives birth between the ages of 25 and 30, then the risk of babies with non-chromosomal birth defects is at its lowest. Women who are both older and younger than this seem to have a greater risk for this type of birth defects
Age and risk for chromosomal abnormalities
The association between fetal chromosomal abnormalities and older maternal age has been widely researched and established. The older a woman decides to have a child, the greater the chances of her baby being conceived with a chromosomal defect such as Down?s Syndrome. This is because a woman?s eggs age as she ages. Older eggs are more prone to forming embryos with either too many or too few chromosomes. This is the reason why older women have a greater rate for infertility, miscarriages and babies with chromosomal birth defects.
Now, however, it has come to the attention of researchers that the extremes of maternal age, meaning women over age 35 and women who are 20 and under, may also be related to non-chromosomal structural abnormalities in the fetus