She was likely closer to 14 when she was engaged to Saint Joseph, and closer to 15 when she bore Christ. In pre-diaspora Jewish culture, girls would not have been typically married until they were old enough to have children, and like Jett implied, girls back then developed later. Thanks to all the hormones and junk in our food today, girls as young as 8 begin developing physically, but on a natural, pre-modern diet 13-17 would have been a more realistic age for a young woman to be able to have kids, and hence get engaged.

The engagement period also typically lasted about a year, after which the young man and woman began actually living together as husband and wife.

This all kind of gets implied in the Gospels. The Annunciation happened after she had already been engaged to Saint Joseph, and she was found to be expecting a Son during her engagement period. This would have had to have been a long enough engagement period for her family to notice she was pregnant. It's also shown that a girl of 12 (Jairus' daughter) was still at home, thought of as child, was not engaged, and was not yet married. So 12 is a bit young.

Based on more natural human development and 1st Century Jewish culture, we can infer that Mary was 13-17 when she was engaged and 14-18 when she bore Christ. thumbs up