Actually lack of one of the B vitamins in diet. Teen girls tend to have babies with Trisomy 21 a lot. Then again what is the first thing a teen girl does if she things she is pregnant? Yes, she diets hard! Not the thing to do if one is pregnant. This is one of the reason pre natal vitamins are high in one of the B vitamins, I just can’t remember which one. It is believed that the B vitamin affects brain and nervous system growth early on in conception. A lack of it, or very low amounts can lead to trisomy 21.
IIRC, odds of a Down’s child increases exponentially with the age of the mother.
The opposite is true.
My local suoermarket hires a few as bag boys.
You never met such kind souls in your life. They are smart too.