This is a very common question and one which is HIGHLY misunderstood. The Texas Family Code section that deals with a child being able to choose a parent to live with is found in §153.009. This is the new statute and repeals the old law found in §153.008.
The old Texas statute stated that a child 12 years of age or older could file with the Court in writing the name of the parent who the child would choose to have the right to determine where they lived, subject to the approval of the court. This DID NOT mean that they could choose which parent they wanted to live with. It meant that they could let their preference be known to the court and the court would decide. This is a common misconception. The choice was simply evidence to be considered with all the other evidence.
As I mentioned above, however, this Texas law has been repealed. What ended up happening is that each parent would get the child to sign the paper to choose them, putting them squarely in the middle of the litigation.
The new Texas law still lets children have a voice or choose, but in a different format. The new Texas law (§153.009) allows a party (a parent) to request the judge to talk with the child. If the child is over 12 years old, the judge has to meet with the child. If the child is under 12, then the judge may meet with the child but is not required to do so. The new law allows the child to have a say so on visitation with a parent as well, if the judge wishes to hear this evidence, not just which parent chooses where they live.
As stated earlier though, this interview is just evidence the court can use to make a decision. The court is not required to follow the child's wishes.
I hope this makes the issue of when a child can choose which parent to live with in Texas a little bit clearer.