For example, is it the coating on the cells that is wearing out?
Actually, the major factor influencing the life span of photovoltaic panels is the corrosion of the silver metal interconnects.
The plastic backing sheet slowly absorbs water from humidity and breaks down to create corrosive acetic acid.
The fastest breakdown is the hot humid (tropics), the least is cold dry (alpine).
This failure mode has been solved for the newest panels that are made with an impermeable glass back sheet instead of a PVA backing sheet.