AFAIK, finding breaks in composite fiber parts is an ongoing topic of research. As noted, it's really hard to know when a fiber part is broken, and this doesn't just apply to paintball tanks. Anywhere these new fangled composite parts are used, particularly hard-to-reach places of complicated life-critical machinery, this problem exists.
The 15-year lifespan is apparently not arbitrary.
The act of hydrostatic testing on fiber tanks technically damages them in some small way. So you can only "test" a tank so many times until it's been damaged beyond a comfortable safety margin. So the lifespan is a multiple of hydro test interval. (15 happens to be divisible by 3 and 5...)
"Accuracy by aiming."
Definitely not on the A-Team.