Most kids with Down Syndrome are born with low muscle tone. Low muscle tone, along with differences in the physical structure of the mouth or tongue, plus other medical complications (such as heart defects, etc) can contribute to the delay of solid food introduction. Additionally, research indicates that kids with DS take longer to progress through the stages of chewing development.
There is a great chart indicating motor/oral motor milestones for feeding development. It can be found here:
Here is a simplified list of feeding developmental milestones:
What I'm excited about is that Josiah currently falls in the 9-12 month milestone category. They delay is more on me, than Josiah. I haven't been as quick to introduce some things as I should have. Having this list is nice. I know what to work on next :)