Does receiving Confirmation make you an adult member of the Church?
The reception of the sacrament of confirmation completes the sacraments of initiation. It makes a person a “full member” of the Church. However, it does not make someone an “adult member” of the Church. After all, in the Eastern Catholic Churches infants receive the sacrament of confirmation at the time of their baptism.
As far as Church law is concerned, canon 97 §1 states that “a person who has completed the eighteenth year of age has reached majority; below this age, a person is a minor.”