There are pre-Islamic customs, Tony, and there are Islamic injunctions. You must not confuse the two. An Islamic injunction is that no female must be forced into marriage and must come of her own volition. Anything other is un-Islamic.
Aisha was untouched by the Prophet (PBUH) when she was very young.
Legal majority is established by evidence of puberty. Unless proved otherwise, boys of 12 or less and girls of nine or less are assumed to be minors. But both are assumed to have reached puberty by fifteen. As soon as puberty is established Muslims become full legal citizens, bound by the ibadah and with full criminal liability for their actions. They may also become married, sign binding contracts and take a full part in the life of the mosque.