I believe the Priest decides if infant or adult
BAPTISM (Cann. 849 - 878) ( edited for size)
Can. 849 Baptism, the gateway to the sacraments and necessary for salvation by actual reception or at least by desire, is validly conferred only by a washing of true water with the proper form of words. Through baptism men and women are freed from sin, are reborn as children of God, and, configured to Christ by an indelible character, are incorporated into the Church.
CHAPTER I.
THE CELEBRATION OF BAPTISM
Can. 850 Baptism is administered according to the order prescribed in the approved liturgical books, except in case of urgent necessity when only those things required for the validity of the sacrament must be observed.
Can. 851 The celebration of baptism must be prepared properly; consequently:
1/ an adult who intends to receive baptism is to be admitted to the catechumenate and is to be led insofar as possible through the various stages to sacramental initiation, according to the order of initiation adapted by the conference of bishops and the special norms issued by it;
2/ the parents of an infant to be baptized and those who are to undertake the function of sponsor are to be instructed properly on the meaning of this sacrament and the obligations attached to it. The pastor personally or through others is to take care that the parents are properly instructed through both pastoral advice and common prayer, bringing several families together and, where possible, visiting them.
Can. 852 §1. The prescripts of the canons on adult baptism are to be applied to all those who, no longer infants, have attained the use of reason.
§2. A person who is not responsible for oneself (non sui compos) is also regarded as an infant with respect to baptism.
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Can. 856 Although baptism can be celebrated on any day, it is nevertheless recommended that it be celebrated ordinarily on Sunday or, if possible, at the Easter Vigil.
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§2. As a rule an adult is to be baptized in his or her parish church and an infant in the parish church of the parents unless a just cause suggests otherwise.
CHAPTER II.
THE MINISTER OF BAPTISM
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Can. 863 The baptism of adults, at least of those who have completed their fourteenth year, is to be deferred to the diocesan bishop so that he himself administers it if he has judged it Expedient.
CHAPTER III.
THOSE TO BE BAPTIZED
Can. 864 Every person not yet baptized and only such a person is capable of baptism.
Can. 865 §1. For an adult to be baptized, the person must have manifested the intention to receive baptism, have been instructed sufficiently about the truths of the faith and Christian obligations, and have been tested in the Christian life through the catechumenate. The adult is also to be urged to have sorrow for personal sins.
§2. An adult in danger of death can be baptized if, having some knowledge of the principal truths of the faith, the person has manifested in any way at all the intention to receive baptism and promises to observe the commandments of the Christian religion.
Can. 866 Unless there is a grave reason to the contrary, an adult who is baptized is to be confirmed immediately after baptism and is to participate in the eucharistic celebration also by receiving communion
Can. 867 §1. Parents are obliged to take care that infants are baptized in the first few weeks; as soon as possible after the birth or even before it, they are to go to the pastor to request the sacrament for their child and to be prepared properly for it.
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Can. 868 §1. For an infant to be baptized licitly:
1/ the parents or at least one of them or the person who legitimately takes their place must consent;
2/ there must be a founded hope that the infant will be brought up in the Catholic religion; if such hope is altogether lacking, the baptism is to be delayed according to the prescripts of particular law after the parents have been advised about the reason.
§2. An infant of Catholic parents or even of non-Catholic parents is baptized licitly in danger of death even against the will of the parents.
Can. 869 §1. If there is a doubt whether a person has been baptized or whether baptism was conferred validly and the doubt remains after a serious investigation, baptism is to be conferred conditionally.
§2. Those baptized in a non-Catholic ecclesial community must not be baptized conditionally unless, after an examination of the matter and the form of the words used in the conferral of baptism and a consideration of the intention of the baptized adult and the minister of the baptism, a serious reason exists to doubt the validity of the baptism.
§3. If in the cases mentioned in §§1 and 2 the conferral or validity of the baptism remains doubtful, baptism is not to be conferred until after the doctrine of the sacrament of baptism is explained to the person to be baptized, if an adult, and the reasons of the doubtful validity of the baptism are explained to the person or, in the case of an infant, to the parents.