16 and 17 year olds have been allowed on the main grid since the Teen Grid closed. AFAIK there have been no reported major problems with them on the main grid since their transition to the main grid in 2011.
Anything younger, well... they would have to be restricted to either PG sims or a special area of the grid with safety measures.
I'm not opposed to the idea - SL has a lot of content you wouldn't want them to see (i.e. Zindra), but MORE content that there is no reason they shouldn't see. Teens are creative and many use Minecraft, so I don't see why they shouldn't have some sort of access to SL's creative tools - even in a restricted and somewhat policed environment.
But if SL is struggling to retain adult users, I'm not sure how many teens would be interested with so many other choices now, such as more and more free-to-play MMO games. And land is too expensive even for most adults now, so many of them would be limited to working in sandboxes.