That's the approximate age that is generally believed to be true; I've seen it it numerous scholarly sources, although I'm not sure if there is a "biblical" source per se.

12ish was the normal age of betrothal for jewish girls at the time. Betrothals usually lasted 1 year. Unlike modern engagements, betrothals were very serious business -- part of the marriage process, not just a promise to wed.

If Mary was betrothed but not yet wed, that puts her age at or about 13 years old (i.e. 12-14), unless she was profoundly different from Jewish marriage customs at the time.