The magic age for deciding where a minor wants to live is 18. Once they're an adult, they can decide where they want to live. Custody decrees are court orders and children are not allowed to decide what the court orders say or if they can violate a court order.
Kids, even teens, often don't understand everything that goes into making a parenting plan. Obviously, courts take into account what kids want when the kids are older and able to really express a reasoned opinion.
It's very common for kids to want to try living with the non-custodial parent for any number of reasons. Life with the non-custodial parent is often weekend-oriented, filled with fun and relaxation. The custodial parent may be financially stressed (paying child support is always cheaper than having a child live with you). A new marriage or relationship may be hard for a kid to adjust to or (if it's the non-custodial parent's) may be really fun and exciting. Teenagers of course are focused more on spending time with friends and may feel closer to friends at the non-custodial parent's area. Unfortunately, the reality of living with the non-custodial parent is usually not a great as visiting the non-custodial parent. And the worst part of it is that the parents have let the child start dictating what happens.
If there are concerns about mistreatment, then get the child to talk to a school counselor or someone else who can report abuse or neglect to the authorities.
Minors really do get to have a voice, but they don't get the final word.