General Information:

This tells briefly about John's growing years.

Now

This word is used here to mark a change in the main story. Luke quickly moves from the birth of John to the beginning of his ministry as an adult.

"became spiritually mature" or "strengthened his relationship with God"

"lived in the wilderness." Luke does not say at what age John began to live in the wilderness.

This does not necessarily mark a stopping point. John continued to live out in the desert even after he started preaching publicly.

"when he began to preach in public"

This is used here in the general sense of "the time" or "the occasion."