How does a person become a godparent in the Roman Catholic Church? Assume a young adult is close to Confirmation in the Roman Catholic church, but, while still a child, never finishes the process because the family moves away from the parish.  What would be the process for this person to become a godparent at this point in the Roman Catholic church? Would it matter if the person in question is the sibling of one of the birth parents?
According to the Catholic Church's Code of Canon Law - Canon 874 §1. states that in order for someone to be permitted to take on the function of sponsor (godparent) a person must:


  1/ be designated by the one to be baptized, by the parents or the
  person who takes their place, or in their absence by the pastor or
  minister and have the aptitude and intention of fulfilling this
  function;
  
  2/ have completed the sixteenth year of age, unless the diocesan
  bishop has established another age, or the pastor or minister has
  granted an exception for a just cause;
  
  3/ be a Catholic who has been confirmed and has already received the
  most holy sacrament of the Eucharist and who leads a life of faith in
  keeping with the function to be taken on;
  
  4/ not be bound by any canonical penalty legitimately imposed or
  declared;
  
  5/ not be the father or mother of the one to be baptized.
  
  §2. A baptized person who belongs to a non-Catholic ecclesial
  community is not to participate except together with a Catholic
  sponsor and then only as a witness of the baptism.


In conclusion - to address your question more specifically, it must be clearly noted that:

(A) The godparent must definitely be confirmed in the faith, both sacramentally as well as proactively.

(B) The godparent must be over the age of 16 unless given a special dispensation by the local bishop.

If this question actually pertains to you or someone you know, my advice would be to contact your local priest...he should have all the answers to questions pertaining to sacraments.