Baptism

In his dialogue with Nicodemus, Jesus taught that Baptism was necessary for salvation. "No one can enter the kingdom of God without being born of water and Spirit" (Jn 3:5). After his Resurrection, Jesus met with the eleven Apostles and gave them the commission to preach the Gospel and baptize, telling them, "Whoever believes and is baptized will be saved" (Mk16:16). 

Who Can Receive Baptism?
Anyone who has not already been baptized can receive the sacrament of Baptism in the Catholic Church. The Catechism of the Catholic Church states that “Every person not yet baptized and only such a person is able to be baptized,” (paragraph 1246). Baptism leaves an indelible (permanent) mark on the soul and there is no way nor any reason that one could be re-baptized.  There are no age restrictions for baptism; you cannot be too old or too young to be baptized.
 

Does the Catholic Church accept baptisms from another church?

What does the Grace of Baptism accomplish?
Yes, the Catholic Church recognizes any baptism that uses water and in which the baptized was baptized was the words “I baptize you in the name of the Father, and of the Son, and of the Holy Spirit.” Some churches do not use the trinitarian formula for baptism and thus their baptisms are not valid.
Catholic Baptism does five things specifically...
  1. It forgives all sins that may have been committed prior to a person’s baptism including original sin, mortal sins, and venial sins, and it relieves the punishment for those sins.
  2. It makes the newly baptized person “a new creature.”
  3. It turns the person into a newly adopted son of God and a member of Christ. Baptism incorporates one into the Church which is the body of Christ.
  4. It brings someone into the flock of the faithful and brings them to share in the royal priesthood of Christ (1 Pet. 2:9-10). Catholic baptism gives a share in the common priesthood of all believers and it also brings about the sacramental bond of the unity of Christians. Paragraph 1271 of the Catechism says it best: "Baptism constitutes the foundation of communion among all Christians, including those who are not yet in full communion with the Catholic Church: “For men who believe in Christ and have been properly baptized are put in some, though imperfect, communion with the Catholic Church. Justified by faith in Baptism, [they] are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers by the children of the Catholic Church. Baptism therefore constitutes the sacramental bond of unity existing among all who through it are reborn.”
  5. Last, but certainly not in the least, baptism leaves and indelible spiritual mark (character) of belonging to Christ on the soul. Nothing you can do will take away this mark even if you sin a million times. Those sins may not grant you salvation, but you will always carry the mark of a Christian on your soul, therefore making re-baptism impossible.