While a D.S. sufferer has intellectual disability, its severity may vary a lot, with some cases mild enough that the capability of giving consent might be there.
Then there are mosaic cases, which are in the middle between a full Down Syndrome and normalcy. In mosaic cases, only a certain percentage of cells is afflicted. People with mosaic aneuploidies can often lead fairly normal lives.
I mean, technically the person born with Down Syndrome would be incapable of consenting on their own life decisions after they were born, right?