Why would someone do this?

Question

If you're a person of authority, and a young adult goes from calling you by your first name when you were younger to your title & last name now? They don't say your name anymore, just in emails.

Answer 1

Maybe it's because they respect your authority now

Answer 2

Did they go a long period of time where they didn't see you? That may effect some of the comfort they have with you. Just laugh it off and casually remind them that they can call you by your first name.