I think I’ve got it!
I’m no scientist, but I checked the MMO Safety Data Sheet and under the heading “Hazard Identification” is a hazard classification reference to “Reproductive Toxicity 2”.
Admittedly not a scientist, but I was once under the age of 18 and if my memory serves me then anything with a toxic influence on reproductive behavior in middle school or high school would NOT be a good thing. If I recall that was a problem without any toxicity to enhance it. 
So, requiring “proof of being 18 years of age,” makes sense to me. 
Sorry, I’ll show myself out…
CSA
