As far as the rules go, her age is determined as of January 1st of that year. So if she will still be 12 as of January 1st you have the option of keeping her in 12u. It has been my experience that kids who tend to be older in their division - as your daughter would be - tend to stand out and do well because a year of development is a lot at that age. If you move her to 14u she will be one of the youngest ones on the team, and therefore may not shine as much. On the flip side - she will have to rise to the challenge. You play with better athletes, you get better.
It's really a personal choice that you and your family have to make. I would take into consideration, not only physical ability but also mental toughness and maturity. If you put her on a 14u team, there are liable to be girls who are actually 15 by the time summer rolls around. Which means they will talk (and have experiences) that is common of a 15 yo girl, that might be a bit much for a 13yo girl.
Just my 2 cents....