Well, I tend to see this as a financially driven decision... LL probably cannot afford to keep two grids operating, and they probably do not want to totally alienate future customers by just chopping off their accounts. So, you merge the grids... what could be simpler?
As for the adult-child interaction issues... doesn't this problem also exist on facebook and other social networks... granted SL is a prime venue for dynamic interactive porn (smiles), but the same issues would be present for static text and image porn on facebook.
.. and what about the access granted via skylight (see: http://blogs.secondlife.com/community/features/blog/2010/11/16/help-us-test-the-sl-web-viewer-beta) .. what about the day when SL becomes a web-based facebook applet?
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And I, for one, welcome our new teen overlords..... It's going to stir the creative pot and lead to change.... and change is a good thing.. for us and for Second Life.
During 2008 and 2009, in my opinion, second life was a stationary product... there were no new server based content creation features and no serious improvements to the viewers. This quiescence led to serious competition from opensimulators and third party viewers. Well, that party is over.. Second Life is evolving again and probably leaving most of those projects in the dust.
des/demona
p.s.: ... and remember.... when you try to cover you ass by asking people "Are you an adult?".. you should probably also consider asking them "Are you a law enforcement officer?" It's my understanding that this is the S.O.P for RL sex workers.