Anything over 15000 msl and you are required to use oxygen before the jump. Itis done quite often but is just too expensive and to much of a hassle to do this for every jump. Plus the jump planes climb slower at higher altitudes so it takes longer and burns more fuel. So 13,500 seem to be a coomon altitude. Some DZ's at higher elevations go to 12,500 and most cessna DZ's only go to 10,500 or 11,500 due to slow climb rates.
Per FAA regulations, above 15,000' MSL, supplemental oxygen is required. Who wants to deal with that on a regular basis? Not me.
Now, because of that 15,000' limit, and the fact that very few places are at sea-level, you get the 13,500' cap.
Let's say that you're at a DZ that has an elevation of 1000'. That means you can go to 14,000 AGL. 14,000 AGL + 1000' elevation = 15,000MSL. MSL = Mean Sea Level, and AGL = Above Ground Level.
There are times when some places will do high-altitude loads, but those are only generally to 23,000'. When you start getting closer to 30K', you have to have bail-out oxygen bottles, special thermal gear, etc... Start going higher than that and you'll need a pressure suit because you can cram all the tank oxygen you want into your lungs, but you can't do anything with it because the oxygen partial pressure is too low.
And let's not even get started with how cold it is above 20K, even in AZ or FL during the summer. That's some cold stuff.
It won't here unless you are at sea level. The FAR's restrict anyone going above 15000msl from doing so with out supplimental O2 availble. Most DZ's outside the mountianous Western US don't carry O2 on every flight.