ArmyMatt,
Is the below the Synaxarion, you referenced? If this is it, doesn't give an age and the Synaxarion from the tenth century. If you have a different version or document, please kindly post the section about Polycarp.
As for the witness of the Orthodox church, do you have anything earlier than the 10th century that gives the age of Polycarp at death or that he was clearly baptized at infancy?
Mind you this is one possible candidate of a named person infant baptized. I will count all the people named people from the early church who were not infant baptized and put up on full list again on the thread.
February 23
Memory of the holy Hieromartyr Polycarp, Bishop of Smyrna (+155)
A disciple of Saint John the Theologian, like Saint Ignatius the God-bearer, Saint Polycarp was consecrated Bishop of Smyrna. He made a voyage to Rome under Pope Anicetus (155-166) who invited him to celebrate the Holy Liturgy in his church and in his presence. When the persecution broke out under Antoninus the Pious, far from showing perturbation, he wished to remain in Smyrna. Finally he consented to withdraw to a small house in the country, a short distance from his city. One Friday around dinner time some soldiers and horsemen set out in search of him. Later that evening they found him at the dinner table. Learning of their arrival he went downstairs, spoke with them, and served them food and drink. He asked them to grant him one hour to pray freely. When the hour to depart had come, he was made to mount a donkey and was led into the city. At the proconsul's injunction, "Curse Christ," Polycarp answered: "I have served Him for eighty-six years, and He never has done me any wrong. How can I blaspheme my King and my Savior? I am a Christian." The holy martyr had an overflow of courage and cheerfulness. Grace radiated on his face. The proconsul was confounded. Then the entire crowd, not able to restrain its fury, began to utter loud cries. They said: "There he is the Teacher of Asia, the Father of the Christians, the destroyer of our gods. Let Polycarp be burned alive." The fire having respected him, the ungodly ones ordered the "slayer" to pierce him with his dagger. Such a surge of blood escaped from the wound that the fire was put out. Then in view of the crowd's opposition, the centurion exposed the body of Polycarp to the eyes of all and burned it. The martyrdom of blessed Polycarp took place on February 23, 155, during the proconsulate of Statius Quadratus and the eternal reign of Our Lord Jesus Christ to whom belongs glory, honor, majesty, and the eternal throne from generation to generation. Amen. (Excerpts from the Acts of Saint Polycarp's Martyrdom).
Fifth Class Feast.