My parents are still close friends with 3 or 4 of the adults who were my Scoutmaster and ASMs as a kid. I'm in my 40s now and I still have to make a conscious effort to address them as anything other than Mr. XXX when I run into them at my parent's functions. In fact, I sometimes find myself just doing the "Catch his eye and start talking" trick to avoid having to use their names.
No issue with first name with adults, and I feel if an honorific is applicable, it is a good idea, but Mr. etc is okay. I do ask my leaders to introduce me to youth as Mr., as I just feel it is the proper method, as I was raised. I had one scout that no matter what, wanted to use my first name for some reason, but ironically he always introduced me as Mr. Like many old guy here, past scouts, some now in their 50's still address me as Mr.