Hi K.-
I don't know what the "magic age" is for worrying about this, but my son is going into 1st grade and having volunteered in his kindergarten classroom multiple times I know that none of his classmates used baby talk in kindergarten. Some obviously spoke more clearly and had larger vocabulary than others but I wouldn't have called any of it baby talk.
Has she had her hearing checked by her pediatrician? Many times speech problems are caused by a problem with a child's hearing. You can have the pediatrician do a routine screen or for a more thorough evaluation you could take her to an audiologist.
I wouldn't let it go until she is made fun of in school. Regardless of what is causing it, whether it's not having a role model around speaking clearly, or if it's a hearing or speech problem, it's not your stepdaughters fault and she shouldn't have to be teased in school about it for things to change.
You could also call the school district to have her evaluated, she may qualify for speech through the school.
Good Luck,
K.