That boot is what the BJ boots looked like on every one of my Explorers, Mountaineers and my ST. I don't know exactly when the OE boots rot away, but I bought all my trucks with around 160k-185k on them. The upper ball joints on my ST were the worst, being extremely worn, squeaky and sloppy.
FYI's: The OE upper ball joints cannot be replaced in the upper control arm. You must replace the entire arm. Moog Problem Solver replacement UCA's come with replaceable ball joints (I don't know about other brands). I've replaced 6 sets of upper and lower ball joints. Not a horrible job, but removing an replacing the UCA bolts on the driver's side is challenging. They only come out and go in one way (lots of fiddling required) and there are A/C and fuel lines that are in the way. An immediate wheel alignment is required after changing ball joints and you may need chamber alignment kits, though only one of my trucks required the chamber kits and that caused me lots of problems because the bolts kept coming loose, ruining my alignment.
Note: If the bushings in the 2-piece passenger side UCA are worn, you can either replace the bushings, or install a 1-piece UCA. If the bushings in your LCA's are worn, you can replace the bushings or purchase complete new LCA's with new ball joints installed.