The features of down syndrome
are dependent on the addition of one extra segment of a 21 chromosome which is more likely as women age .
You can get this several ways. The traditional DS (~95%) has 3 copies of a 21 chromosome. About 4% have the specific extra 21 piece stuck on a regular chromosome (trans-location).About 1% have a mosaic of some normal and some 21 trisomy in variable amounts.