The tent is going to hell in a handbasket pretty quickly. Its ultimate demise is uncertain, of course, but my guess is that after 15 years the tear strength of the fabric is about half what it once was. The reason for this decay is, as you note, UV radiation from the sun. So, it's always prudent to pitch a tent in the shade, and ALWAYS use a fly in order to shield the canopy (a fly is cheaper to replace, provided the tent in question is still made). Some tent makers use polyester material in their fly construction, which allegedly resists UV better but also will eventually succumb.
A sleeping bag is a very different matter, for two reasons. For one, it's usually kept out of the sun. So, while it's true the nylon is deteriorating, it's doing so at a MUCH slower rate than tent fabric. A good quality down sleeping bag should easily last 20 yearsthe down inside will go to seed before the shell will. For another thing, the sleeping bag shell has a much easier task than the tent fly. All the shell has to do is keep the down contained; not something that really stresses it. The fly, meanwhile, is stressed just from set-up, then also has to take uneven force from gusting winds and all Mother Nature can throw at it. So it leads a short, miserable, and brutish life, while your sleeping bag shell lounges inside, snug and warm, hoping only that you wear long underwear so your greasy skin doesn't rub against it.