At what point does a child, in particular a girl, become responsible? Is it with the first menstruation or with other signs of puberty like hair under the arms, budding breast, etc. Also what is the minimum age?
A young girl is considered legally responsible (mukallafa) once she either has a wet dream, or when coarse hair (as opposed to thin soft hair) grows on the pubic area, or by menstruation, or by becoming pregnant (even if she was not known to have a menstrual cycle). Some scholars include among the signs also the augmenting and splitting of the nose, the bad underarm odor, and the increase in the size of the breast. And if none of these signs appear, she is considered legally responsible once she reaches 18 lunar years.
source : http://www.lamppostproductions.com/about-female-maturity/