It is more that they want people who are actually serious about finding and fixing bugs, rather than people who just wanna play the new content early. One must draw a line somewhere, and they decided to draw it at 16. I'm kinda surprised it is 16 and not 18 to be honest.
Actually, I don't think this is completely true. As Re-Logic has stated with for instance the search for forum staff, maturity is what they're looking for, regardless of how old the possessor is (although I presume there is some bottom line). I think an important factor in this decision is legal stuff, such as NDA's being able to be declared void if the signer is a minor in case of a breach (not sure how/if that works in the U.S., but it's possible where I live), and also not being able to sue minors (again, depends on the location).
With that said, I'm not the Re-Logic recruiter, so what you said might be true after all.