The most common ones in American English, for describing a birthday that hasn't yet arrived, would be:
She turns 12 in July
She will be 12 in July
She will turn 12 in July
She will be turning 12 in July
She's going to be 12 in July
She's going to turn 12 in July
She's going to be turning 12 in July
This is because you get your choice of two future tense markings ("will" or "is going to"), two aspects (simple or continuous) and two different verbs ("to be") or ("to turn").
However, be aware that "be" and "turn" have slightly different meanings in this context. "To turn" unambiguously refers to her birthday; "to be" does not. So, for example, you could say:
You have to be 12 to join, and she's only 11. But it should be okay, because her birthday is in May, so she will be 12 in July, when the course starts.
You could not use "turn" in the boldfaced portion of that sentence. However, you're correct that when used on its own, without a surrounding context, "will be 12 in July" strongly suggests that her birthday will be in July.
You can also, of course, use a large number of other expressions to express the same idea:
Her twelfth birthday will be in July
July will be her twelfth birthday
She'll be eleven until July
and so on.