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. Those who were baptized in another Christian denomination need not be re-baptized so long as their baptism is valid (baptized with water in the name of the Father, and of the Son, and of the Holy Spirit)

There are no age restrictions for baptism; you cannot be too old or too young to be baptized.

Adapted from: aboutcatholics.com

Some converted to Catholicism from another Christian denomination may not need to be baptized. Baptizing someone twice is not necessary so long as the person was baptized in water using the Trinitarian formula. The only way someone could be “baptized twice” is if the original baptism was not valid. However, this would not actually be a second baptism because the first “baptism” was not a real baptism. Most Protestant denominations have a valid baptism. 

The Trinitarian required for Baptism is:

I baptize you in the name of the Father, and of the Son, and of the Holy Spirit.

Adapted from: aboutcatholics.com

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 a person 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 least, baptism leaves an 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 prevent you from being open to the salvation God offers through baptism, but you will always carry the mark of a Christian on your soul, therefore making re-baptism impossible.

A godparent's role is to take over or assist in the faith development of the person being baptized. This is especially important in the event that the parents cannot or do not adequately raise their children in the faith. Godparents can also be helpful through their prayers and example even if the child’s parents fulfill their own promise to raise the child in the faith. Therefore, the faith of the godparents is important in fulfilling the baptismal promise of being raised in the Catholic faith.

Adapted from: aboutcatholics.com