I'd say it's something like: wear = (mileage*average duty level) + (age*environmental factors)
My expy is a 2011 and spent its whole life in Phoenix. I picked it up with 155k miles and it was the kid carrier for a large family since 2013. Lots of road trips and in-town driving so this thing had only seen nicely groomed roads. However, the shocks in that rig made it feel like boat on semi-calm waters even on flat roads. That seal for the piston rod probably got broken down over time due to the constant regional heat plus the valves within it had seen better days after hauling around an 8 person family for nearly a decade. Also, don't forget, the bottom of the piston rod where it connects to the LCA has a rubber bushing in it.
If you don't mind doing a little wrench swinging, I'd recommend swapping them out for new ones since you're asking the question. That means you're probably going to be doing it this year or next year anyway haha. Might as well enjoy the nice ride sooner rather than later!