"Even from birth the wicked go astray. From the womb they are wayward, spreading lies." Psalm 58.3.
Haven't you heard about Esau grasping Jacob's heel before he was born?
Any why on earth would you think someone with Down's Syndrome can't sin? They also have an ability to choose good or evil.
ALL humanity other than Christ is fallen. Jesus is the only person who doesn't sin. Because only God does not sin. And that's why only He could redeem us.
Do you understand the distinction between Original Sin and personal sin?