I'm not sure there's really a reasonable limit, but keep in mind that after 10-15 years most rubber parts have deteriorated such that some parts may need replacement. And that's not just belts and hoses, but suspension bumpers and various other items. However they're replaceable, but failure may catch you unprepared some time. Fabric items (awnings, etc.) and general weathering may also be a problem.
Otherwise, features and condition, including chassis type and construction, overall condition (including maintenance of various things), and general care of the unit are things to note. Higher end coaches probably last longer, at least to a degree, due to better materials and, perhaps, better design, but it's basically a case-by-case basis.
All that being said, a floor plan you will enjoy (not just something to "live with") and your budget (not just purchase budget, but for maintenance too) will be major factors. Still, all else being equal (a rare case indeed), a newer unit will probably be more reliable than many older units.