>>3841857112 is the age they basically have developed secondary sex characteristics (they develop from 8-12 IIRC) then later they just grow. At 12 a girl is already a woman physically, and yes they can bear children (there are even innumerous cases in asian/african countries every day, just search it), but of course it's too risky, specially now that we normalized C-sections and tons of women can't even give birth naturally. About menarche, the average age has been 12 since for a long time now.
