It depends, how does the adult want to be addressed?
Growing up, we had a LtCol. in various positions. He was also a MD. We addressed him as "Doc _____________" or just plain "Doc"
In my middle son's den, his DL was an MD, but went by "Mr. _______________"
Adult to adult --- First name
Scout to adult -- Mr./Mrs./Ms. No professional honorifics
Adult giving instructions and referencing another adult -- Mr./Mrs./Ms.
I don't like it when kids address adults without a courtesy title, but that might be more a reflection of our demographics. Some of the kids around here could use reminders about basic manners and social courtesies, especially in formal communications.
I’m a first name guy. Period.
But, that’s not the culture I live in. So, if scouts are compelled to address me as Mr. Q., I try to address them as Mr./Miss. My problem at the moment is our troop is chock full of siblings, so I’m using first names more regularly.