Assuming it's a genuine effect, it could be due to other factors: for example nightshift work is associated with reduced lifespan and increased cancer rates, apparently because of disrupted circadian rhythms - and flying across timezones is pretty darn disruptive too.
This could be tested by looking at people who fly a lot in a north-south direction (if there are any), who would get cosmic rays but no jet lag. In any event, the flux of cosmic rays is pretty much known, so the total exposure such people get can be computed and compared to that of radiation workers etc.
7 Random Things You Won't Believe Are Shortening Your Life
TL;DR
1. Starting School Early
2. Being the Eldest Child
3. Having a Small Ass
4. Flying Often
5. Living Closer to Sea-Level
6. Not Going to Church.
7. Retiring Early.
I'm glad a high-traffic site such like Cracked is giving some exposure to the issue. Can anyone here comment on the reliability of their sources?