Many interesting opinions on this thread. Mostly just that, opinions.
Many dealers adopted the 10 year rule 20 or 25 years ago when something over 10 years old was really a quivering pile of parts.
If you could have seen what most 10 or 15 year old bikes looked like back in 1983, you'd understand.
Now though, the bikes (mostly Hondas) are designed to last longer with less service. And come apart and go together easier.
I was lucky many, many years ago to work in a Hon/Suz/Kaw/Duc shop for a great Service Manager. He didn't have an iron clad rule about not working on old bikes. We worked on almost anything.
But, when he went out to look at someones pride and joy, the estimate would be a little higher for an old, rusty looking bike that wintered outside, chained to a tree. If the bike staggered under 200 lbs of add-ons that had to be removed or worked around for normal service, well...you know.
The worst was someone coming in with a 20 year old wing with 30 additional lights on it, and all the extra wiring was one color. Whatever was on sale at the local NAPA when a roll of wire was needed. And crappy crimps on the terminals that are stacked 20 deep on a longer battery hot post bolt!
It would have been easier to have a 10 year rule, but several of us had older bikes and didn't mind working on the older ones. He let the nicer examples through the door, even though it took a little longer to do individual assessments.
This was back in Ohio in an earlier life, and Ohio has a law that says the job quoted can't go over 10% of the quote without an O.K. from the owner. We would stop and get an O.K. on anything that would push the total up at all. Little stuff just got fixed and glossed over. We were flat rate techs with no time for frivolous phone calls. It had o be important for us to stop and call.
Well, my Ibuprofen and Aleve cocktail has kicked in. I gotta go fix the wifey's washing machine so I can put the new fuel pump in my '95 GL1500. Then I can get it off the rack and get ready to change out an engine on one of my Ducatis. Gonna be a busy week.