Yes, 35 was the measure, yet I’ve met younger and older mothers of down syndrome children. I know a family whose first child had downs and the younger 4 didn’t. So while it seems age plays a factor, it certainly isn’t the only factor.
In my own family - My mother had 6th child at 42. One niece had 1st child at 40, 2nd at 43. One niece had 1st child at 43. No problems. All very bright healthy children. I expect that when families had 10-12 kids over a period of 20 years - this might not have been so unusual.