How did you determine that she was 12 or 13? I've found that most of our ancestors had no idea how old they were. Ages that we deduce for the same person vary >from record to record. So, unless you've actually got a copy of her birth record that proves her age, she really could have been several years older when she got married.
Birth records are not necessarily accurate either. Months or years could go by before a birth was registered. It is not unusual to find 2-3 consecutive birth registrations of siblings who weren't twins.
