Teeth can be tricky. What you need is a pic of the occlusal (biting) surface of the teeth, a pic of the "corners" (3rd incisors) and a profile shot to see the angle at which the teeth come together. We put all these pieces together to judge an age. Things like dental care and cribbing can through a hitch into getting an accurate age but shouldn't throw you off by more than a few years. Galvayne's groove has been shown in multiple scientific studies to be wholly inaccurate and I never use it when trying to age a horse.
I guess I'll just have to go with my gut instinct on her age.