Wa alaykum salam wa rahmatuLlahi wa barakatuHu,

In the Shafi’i School, a girl must mature to an age where she is ready for relations, prior to that it is unlawful. One must note that relations may well harm a girl at a young age; thus, waiting until she is physical and mentally ready is necessary. The possibility of inflicting harm on her must not exist. These two considerations, 1)  the absence of harm and 2) her readiness, are what have been stressed in the Shafi’i School pertaining to when relations are lawful. (See Rawdat al-Talibin v. 7, p. 261; Mughni al-Muhtaj v. 4, p. 373)

And Allah knows best.