Q. 1. Has the Vatican released a document that establishes the minimum requirements for a person to be an Altar server? For example, is there a minimum age? Should a child with Down syndrome be allowed to serve when it is obvious that particular person cannot properly serve as an Altar server?
A. 1. Regarding the minimum age of an Altar server, such varies based on local circumstances such as a Bishop's decree, tradition, the culture, etc... The important thing is that the child, boy or girl, must be mature enough to carry out his/her duties without disrupting the sanctity of the altar.
Regarding one's service at the Altar, no one is really worthy to serve in the House of the Lord, but some are capable. Other are unable to do so. A child with Down syndrome would not be able to properly carry out the tasks of an Altar server. As such, this inability would disqualify him/her from serving at the altar.
In the Book of Leviticus, we read that the Lord God commanded than anyone with defects "must not go near the curtain or approach the altar, and so desecrate his sanctuary." [Leviticus 21:16-23]