1. a person with Down Syndrome
2. any mentally retarded person
A reference to the fact that people with Down Syndrome have three copies of the 21st chromosome (hence the other name of the syndrome, trisomy 21), while normal people only have two. Therefore, while most people ride "bicycles" down "21st street", people with Down Syndrome ride "tricycles." They are trike riders.
Check out that kid over there. He's definitely a trike rider.