Soooo, I'm in a similar situation as OP here.... I got my cousin into brewing about 5 years ago. He brewed 2 or 3 beers, but then more or less quit. All of his equipment (extract kits, buckets, plastic carboy, bottling wand, capper, etc) has been stored at my house for this entire time. I see him every now and then, but he never mentions any of it, and hasn't had any real inkling to get back into it.
I, on the other hand, have probably brewed 200 batches since then, gone all grain, competitions, blah blah blah...
He brewed a 5 gal extract batch of a Sierra Nevada clone (NB? Midwest? I can't remember) that has been sitting in my brew closet now for about 5 years. At some point (likely before the airlock dried out) I replaced the airlock with a carboy cap. This carboy hasn't seen oxygen or light in those 5 years, nor has it been moved more than an inch or two. I've debated dumping it, I've debated kegging it, and I've feared both.
I haven't opened it. It's still on the yeast cake and the carboy is plastic - so I fear that enough oxygen has made it through the walls at this point and the yeast might have done weird things to it (I think it's called autolysis?). This thread has made me think about checking that carboy out....