The certification of both games is slightly different. Black ops is an 18 certificate, just like a film, whilst Halo reach is a PEGI 16. PEGI is an indicator of the content and the age range it is intended for. It is not a legally enforced certification. Black ops is a certificated 18 title and as such, the content is intended for 18 and over.
I have the joy of dealing with ignorant parents who coldn,t give a damn that their kids want 18 cert games despite not knowing the content or why it is an 18. Most just say their friends also have them and still buy it to keep them quiet. The debate on content and the effect it has will go on, but if parents dont give a crap then the kids wont either.
My daughter is 9, and has watched the odd 12 cert film (after we have checked) but i won’t let her see a 15/18. I guess i’m a little old fashioned.