Clearly St. Anthony was not alive when Jesus was a baby? I’ve always wondered this!!!
There is no definitive reason, it didn’t become common until the 17th Century.
One version is that Jesus appeared to St. Anthony in child form near the end of his life. Count Tiso, who provided a chapel and hermitage to the friars in the area saw this, but St. Anthony asked Count Tiso not to reveal this event until after his death.
Another version is that St. Anthony is portrayed that he was was a follower of St. Francis who experienced a miracle. St. Francis had people come to Midnight Mass in a cave where there was an ox and an *** and a manger filled with straw. And the story went around that the Christ child appeared in the straw and Francis held the child in his arms. St. Anthony with infant portrays the message that he adhered to the theology and teachings of St. Francis.
Another version, and I like this one the best. In John 1, we read about Jesus being the Word, and the Word is God. St. Francis preached the Word, and rather than depicting the Word as a bible, or scroll. They used the image of the infant Jesus.
I like this best because in a way that means that every priest and religious who preaches or has preached the Word also has held the infant Jesus.