The average age for individuals to be diagnosed with Mosaic Down syndrome is 1-4 years of age. The most common reason for this late diagnosis is the lack of knowledge in the medical field on this rare form of Down syndrome. However, many individuals can go undiagnosed up into adulthood and there are still thousands who never receive a diagnosis. Mosaic Down syndrome can be found in 1-2% of individuals and is where some of the cells in the body are normal and other cells have trisomy 21. For more information, you can visit the International Mosaic Down Syndrome Association.
The extra chromosome 21 material that causes Down syndrome may also be due to a Robertsonian translocation, referred to as Translocation Down syndrome, which is when the long arm of chromosome 21 is attached to another chromosome, often chromosome 14 or itself. For more information, you can visit the NDSS website.