Perhaps consciousness is not a crisp yes/no choice but a gradient... Dolphins are conscious but to a degree less than humans, likewise mice less than dolphins and so on.
Perhaps, and perhaps a person with Down's Syndrome has a lesser consciousness as well, but we have no way of knowing that. Personally I think it is pretty arrogant and shot-sighted of us to think that our experience of the universe is somehow better and more real than anything else's.