It is very unusual. Most parents will find out during pregnancy, immediately after birth, or within a few weeks of birth.

In our case, somehow none of the health professionals noticed or said anything (yep, we seriously found out later that at least 3 of them suspected and said nothing - because it "wasn't within their remit"). Typical Down's facial features are not quite as prominent in our daughter, and developmentally, for a long time she tended to match her peers, or only lag behind a little. But coming on for 2 years, she still wasn't walking and her speech was lagging being, and we went to see a paediatrician - he recognised it immediately, and told us without even waiting on karyotyping. It was something of a shock :)