Hi,
I would say model years 2005 and earlier are the ones to be concerned about. There is a tag on the motor mount that details the month and year of manufacture.
It is very easy to scope the motors, all you have to do is drop the lowers. They stick a camera on a stick up the mid section of the motor and inspect the exhaust, the mid section coated housings and the plate between the power head and the exhaust. I did this for 3 years in a row during winterization. The pictures showed what I'd call slight progressive damage. I decided after 8 years and ~600 hours to do the work, as the pictures showed corrosion. I had 7/2005 dated F250s motors and had both torn down and the mid sections torn down and the damage was far less than the camera shots led me to believe. I think this has to do with the conditions the pics were taken. It is a tight dark place and the corrosion looked worse than it actually was. A total guess on my part, but I think I could have easily gotten 4 or more years. There was corrosion and some salt deposits where the coating was no longer there, but not massive amounts.
Since the engines were torn down and I had all the parts in hand, I did the job. I also replaced the oil pumps, speedy crank, internal anodes and a bunch of other recommended work by Sim Yamaha and others. It wasn't cheap, but they were complete (piece of mind) and they ran great last year (first year of use).
I'm told Yamaha has a heat map of the failures. Where they map the location and frequency of corrosion and it is concentrated in water with high temperatures - Carolinas and more so Florida. I didn't see the map as I'm told only Yamaha and their dealers that ask see it.
What to do? Get them scoped. Look for more than shiny spots - look for demonstrated evidence of metal being corroded. Nobody seems to know how aggressive it gets once the coating is compromised. All you can really do is scope them every year and compare pics.
Good luck. PM me if you have questions and I may be able to dig up some pics to send via email.