there are 3 reasons why I agree w/this rule-
1) as someone who started smoking ciggarettes when they were 11yrs old, and deeply regrets it, I can tell you that there are some decesions that are better made by well informed adults. And as a father of 2 underage children, I would preffer that they waited untill they were older before making such decissions.
2)hypothetical-Let's say little "Billy" gets caught useing banned substances by mom and dad. They search his room and find his $25 chinese shisha and the Hookahpro tab on his PC. Next thing you know, Sam has to deal w/messages from irrate parents and so goes the perpetuation of the stigma that many of us work so hard to dispell.
3)Personally, like many members, I'm well into my 30s and use this forum as a place for mature, informed discussion of an ancient hobby that many of us take quit seriously. I don't necessarily want to constantly scroll through threads about how cool it is that "billy" can blow smoke rings through a toilet paper tube!
PS: I just woke up and have'nt had my coffee yet. Sorry if I sound cranky!