In my opinion it is much more a matter of physical maturity than it is age. Lifting a significant amount of weight can be damaging to the growing body.
The NCEP Bronze book suggests that kids beginning at age 12 can begin lifting light weights. They encourage, focusing on technique, using broomsticks or bars alone.
There are other ways to add strength without adding weights too early. Doing decline pushups, doing incline situps, etc.
