You need to have registered your account at the birth-year that calculates to you as being 18+ at this very moment.
Blood Effect will be an option to disable or enable if you've signed up as 18 years old +
It won't be an option, but fully disabled if you signed up under 18. I believe it'll follow you as you grow too.
so if you signed up two years ago as 16 years old on June 10th 2009
then June 10th 2011, (possibly or a few weeks after) you'll be able to use the Blood effect.
It's not really necessary because it does cause slight lag, but hey Combat Arms is always laggy, yet it does help when you're determining whether you shot the guy or not... also a great look to those montage-worth headshot clips.