Regarding the history of Pentecostalism, isolated Christian groups who were experiencing charismatic phenomena such as divine healing and speaking in tongues, eventually came together. The holiness movement provided a theological explanation for what was happening to these Christians, and they adapted Wesleyan soteriology to accommodate their new understanding. It was not until the early 20th century that the denomination was established under the banner of Pentecostalism.
Pentecostalism or Classical Pentecostalism is a renewal movement within Protestant Christianity that places special emphasis on a direct personal experience of God through the baptism with the Holy Spirit. It is distinguished by belief in the baptism in the Holy Spirit that enables a Christian to live a Spirit-filled and empowered life. This empowerment includes the use of spiritual gifts such as speaking in tongues and divine healing—two other defining characteristics of Pentecostalism. Pentecostalism emerged in the early 20th century... In 1900, Charles Parham, an American evangelist and faith healer, began teaching that speaking in tongues was the Bible evidence of Spirit baptism and along with William J. Seymour, a Wesleyan-Holiness preacher, he taught that this was the third work of grace. Source: https://en.wikipedia.org/wiki/Pentecostalism
Although many of the Christians came out of churches that adopted infant as well as adult baptism by water (Anglicans and Methodists, for example) not all of them did (Baptists).
Believer's baptism (occasionally called credobaptism, from the Latin word credo meaning "I believe") is the Christian practice of baptism as this is understood by many evangelical denominations, particularly those that descend from the Anabaptist and English Baptist tradition. According to their understanding, a person is baptized on the basis of his or her profession of faith in Jesus Christ and as admission into a local community of faith.
The contrasting belief, held in nearly every other Christian tradition, is infant baptism (pedobaptism or paedobaptism, from the Greek paido meaning "child"), in which infants or young children are baptized if one or both parents are already members of the denomination. Such is the practice in the Roman Catholic Church, Eastern Orthodox Churches, Coptic and Oriental Orthodox Churches, Lutheran Churches, Anglican and Episcopal Churches, and others.
Baptisms are performed in various ways: believer's baptism is typically only by immersion or pouring (also called affusion) and infant baptism by either immersion, affusion, and aspersion (sprinkling). Believer's baptism is often referred to as adult baptism due to the denial that faith can exist prior to the age of accountability. Believer's baptism is also often extended to children so long as they are old enough to earnestly profess their faith. Source: https://en.wikipedia.org/wiki/Believer%27s_baptism
See also: https://en.wikipedia.org/wiki/Infant_baptism
However, in order to establish WHEN infant baptism may have been rejected by the Pentecostal movement, it would be necessary to first find documented evidence that showed they ever embraced or practiced infant baptism in the first instance. I have searched in vain for any reliable source that proves there was a moment in time when either the founders of the Pentecostal movement or the leaders of the various denominations within Pentecostalism denounced infant baptism and said they would no longer perform it. That does not mean to say such evidence does not exist – only that it is impossible to identify at which stage in the development of Pentecostalism infant baptism was rejected if it was never embraced to begin with. Pentecostals identify three distinct uses of the word "baptism" in the New Testament:
Baptism into the body of Christ: This refers to salvation. Every believer in Christ is made a part of his body, the Church, through baptism. The Holy Spirit is the agent, and the body of Christ is the medium.
Water baptism: Symbolic of dying to the world and living in Christ, water baptism is an outward symbol of that which has already been accomplished by the Holy Spirit, namely baptism into the body of Christ.
Baptism with the Holy Spirit: This is an experience distinct from baptism into the body of Christ. In this baptism, Christ is the agent and the Holy Spirit is the medium.
Pentecostals believe that the biblical pattern is "repentance, regeneration, water baptism, and then the baptism with the Holy Ghost". The ordinance of water baptism is an outward symbol of an inner conversion that has already taken place. Therefore, most Pentecostal groups practice believer's baptism by immersion.
After searching various official Pentecostal web sites, I found this article of faith on baptism:
WATER BAPTISM: The scriptural mode of baptism is immersion, and is only for those who have fully repented, having turned from their sins and a love of the world. It should be administered by a duly authorized minister of the Gospel, in obedience to the Word of God, and in the name of our Lord Jesus Christ, according to the Acts of the Apostles 2:38, 8:16, 10:48, 19:5; thus obeying and fulfilling Matthew 28:19.
The basic and fundamental doctrine of this organization shall be the Bible standard of full salvation, which is repentance, baptism in water by immersion in the name of the Lord Jesus Christ for the remission of sins, and the baptism of the Holy Ghost with the initial sign of speaking with other tongues as the Spirit gives utterance.
Given the emphasis placed by Pentecostals on baptism by the Spirit, the resulting speaking in tongues and other gifts of the Holy Spirit, and the official view of adult, or believers’ baptism, it seems doubtful that infants would have been sprinkled with water in some form of infant baptism.