I guess I technically fall in the 35 and under (for 3 more weeks anyway) so that might color some of my opinions. By the way this is my first post here in a long, long time. I couldn't even begin to think of what my ID and password would have been back then.
It's funny, the only troop that I have ever been in (and I have been in 7 so far) that required MR_Whatever was the troop where I was stationed in Germany. It was disconcerting to me that as an 18 year old to be called MR by 15 and 16 yr olds. I didn't have a problem with the 11 yr olds calling me that but it did bother me with the older scouts. I think that dislike has affected me every since.
The troop I am in now definitely does not require last names, by any stretch. The SM is around 68 and has been the SM for around 40 some odd years. He has many of his former scouts come back with their own kids to the troop and he is still just Dave, not Mr_Whatever. He is probably the most committed scouter I have ever met in my life and if he can live with it, I figure thats good enough for our troop.
We have had issues over the years with new adults who have come in to the troop wanting to be called Mr_SoandSo, and can't understand why myself and the other adults don't want to. Usually they have just crossed over from cub scouts with their sons (and I can see why they want it there). It's tough on the boys to have to call 1 of the 5 adults Mr, but not the other 4. Some of the adults have switched over to first names, others have gone on to other things because they couldn't handle the informality of the troop.
We had one Scout in the troop who called everyone in any kind of authority Sir, even patrol leaders. At first the veteran in me liked it, but it didn't take long to see that there were other issues at work, and it was definitely ostracizing (sp?) him from the other Scouts. But his dad was a different kind of ex-military than me and his son was going to call everyone sir and that was it. As someone else pointed out you can't force anyone to raise their kids a different way, even if it is causing them emotional damage.
To me a title isn't respect. I can heap a great deal of derision on a person while still calling them MR, or Sir (been there, done that). In the military you are required to call officers Sir, respecting the rank if not the person. That was probably one of the reasons I didn't last long in the military was having to give that title of respect to somoene I thought didn't deserve it. To me Scouting is the same way. If the boys respect you they will show that respect in many ways. If they don't respect you, forcing them to call you mister just because of your position is not going to prove anything.
The only time anyone calls me Mr is when I am in trouble. :-)
My .02
Ted McLaughlin