I was just about to write and print a small poster for a month-old baby as he is about to be baptized soon. I wanted to put a "Happy Baptism [name]" when I felt the urge to double-check because it didn't sound right. And it turns out, "baptism" is not the right description of the event but "Christening".
What is the etymology of why this split occurred between the event for a baby and the similar event for an older child / adult?
The following entries on the Online Etymology Dictionary say that the terms are equivalent, but don't say how the terms are used differently at different ages.
Christen (v.) c. 1200, from Old English cristnian "to baptize," literally "to make Christian," from cristen "Christian" (see Christian). General meaning of "to name" is attested from mid-15c. Related: Christened; christening. - etymonline
baptize (v.) c. 1300, from Old French batisier (11c.), from Latin baptizare, from Greek baptizein "immerse, dip in water," also figuratively, "be over one's head" (in debt, etc.), "to be soaked (in wine);" in Greek Christian usage, "baptize;" from baptein "to dip, steep, dye, color," from PIE root **gwabh*- "to dip, sink." Christian baptism originally consisted in full immersion. Related: Baptized; baptizing.