I'd like to point out here that menstruation does NOT equate to physical maturation (puberty), for females. Menses signifies the ability to conceive-- not the ability to carry, give birth, or feed the offspring.
These practices are disgusting. Children cannot fully comprehend any decision like marriage at this age. Even if she had a choice, it wouldn't be truly her's to make.
Exactly.
Even if a young girl has started her periods her body isn't necessarily physically ready to handle the stress of actually giving birth and as has already been pointed out 12 and 13yr-olds having consensual sex (as messed up as it is) in the West is not the same as being forced or coerced into marriage and sex.