I started trying when I was 34, got pregnant at 35, and had DS at 36. I remember going through prenatal testing feeling grateful that I got pregnant at 35 instead of 34, because the increase in risk is negligible but the increase in care is substantial. Doctors and genetic counselors told me that the "advanced maternal age" of 35 is a rather arbitrary choice, since 35 is not that much riskier than, say, 33 or even 31, but they have to draw the line somewhere and that's what's been chosen. The biggest difference is the willingness of doctors to order the full gamut of tests and the willingness of the insurance companies to pay for it. By the time I was 13 weeks, I knew the fetus was negative for all chromosomal abnormalities, negative for Cystic Fibrosis, some other things that escape me and, bonus, had a gender confirmation. It gave me great peace of mind for the remainder of the pregnancy and I remember thinking how much it would suck to be a young 22 year old and not having access to this level of care, possibly only finding out my baby suffered from one of these conditions at the birth. Of course, no one has to do testing to that extent if they don't want to, but at 35+ it all becomes available to you without hassle or expense. So for every bit of increased risk that comes with age, I felt it was more than mitigated by an increased level of care.
My personal cut off for having more children will be when my husband and I decide we don't want more or when my fertility runs out, whichever comes first, but it won't be based on an arbitrary age.