I have a cousin that had downs. We were told by the parish priest that it would be his decision to determine whether or not she understood what she was recieving. If he could communicate with her enough to determine that she did indeed understand, then she would be able to make her first communion. Unfortunately she didn’t get the chance as she died of a heart problem. I believe the teaching is based on each individual reaching the age of reason – that will be different for different people, typically they say 7-8 years old.