"Callow" is an adjective meaning immature or inexperienced. To express the act of being callow, you would use verbs that describe immaturity or inexperience, such as:
* To be naive
* To be inexperienced
* To lack maturity
* To be green (informal)
* To be unsophisticated
For example, instead of saying "He was callow," you could say:
* "He was naive about the world."
* "He lacked experience in handling such situations."
* "He was green behind the ears."
You could also use verbs that describe the process of becoming mature or experienced, such as:
* To mature
* To learn
* To grow
* To gain experience
For example:
* "He matured significantly after spending time in the real world."
* "He learned from his mistakes and gained experience."