When I was a newly-minted Ph.D, I made a point of having my students use the title because I wasn't much older than they were and I wanted to recognize explicitly that while we were all adults, there was a power imbalance in our current situation. I could be friendly and approachable, but at the end of the semester I would need to assign them grades, and I was getting paid to be there while they were paying.

As I got older and they didn't, reminding students that we really weren't equals in this context became less of an issue.

Now I'm in the reverse situation: I'm at a workplace where everybody goes by first names, and at least four of my former students work there too; it takes work to get them to use my first name.