SL already makes a killing from adults, in a surprisingly broad age range. If anything, teens probably spend comparitively less money there. The teen grid was an attempted no-dildo zone to placate the "think of the children" crowd.

Note that the main grid and teen grid are completely segregated.. if you're allowed on one, you're not allowed on the other. The teen grid is completely moderated and "education-focussed". I'm sure it's insanely boring.