I have a son with Down Syndrome. I wondered the same thing when he was born - if he would be able to have a "normal" life and do things such as serve a mission or marry in the temple. Since then I've learned that determining whether or not someone with a disability can/will serve a mission, really depends upon the individual and their personal abilities, level of comprehension, spirituality and other circumstances - just like any one else. The same applies to baptisim, receiving a patriarchal blessing, receiving the priesthood, or receiving their temple endowment. I heard last year, of a boy (well, he's a 25 year old man) with Down Syndrome who was getting married in the Bountiful temple! I've personally been taught by a missionary with disabilities. There is a boy in my mom's ward (President Monson's home ward) who has Aspberger's Syndrome, and blesses and serves the sacrament. Although I haven't really looked, there may be more information on the new church website specifically for people with disabilities and their families: Disabilities Home Ultimately, this is something your family and the family of your cousin should pray about, and discuss with the bishop, and determine if your cousin as an individual should be called on a mission!
