From what I remember from school/uni Downs is associated with mothers age and the quality of her eggs. As she gets older the quality of eggs deteriorates hence the higher risk at older age. There may very well be a predisposition if there is family history.
Passo, I don’t think anyone is saying if one doesn’t personally want to get married to a cousin they are defying Allah (SWT) naouzbillah. I personally wouldn’t either. I just think people should make informed choices.
Yes, there is a genetic component to cardiovascular disease but its extremely common in South Asians so doesn’t matter if someone is your cousin or not. Since when do people inquire about diabetes and hypertension before getting married?