Legal Question in Family Law in California

They don't get to choose until they are 18. They have a legal right to be heard in court at 14. The law on this is based on Family Code section 3042:

3042. (a) If a child is of sufficient age and capacity to reason so

as to form an intelligent preference as to custody or visitation,

the court shall consider, and give due weight to, the wishes of the

child in making an order granting or modifying custody or visitation.

(b) In addition to the requirements of subdivision (b) of Section

765 of the Evidence Code, the court shall control the examination of

a child witness so as to protect the best interests of the child.

(c) If the child is 14 years of age or older and wishes to address

the court regarding custody or visitation, the child shall be

permitted to do so, unless the court determines that doing so is not

in the child's best interests. In that case, the court shall state

its reasons for that finding on the record.

(d) Nothing in this section shall be interpreted to prevent a

child who is less than 14 years of age from addressing the court

regarding custody or visitation, if the court determines that is

appropriate pursuant to the child's best interests.

. . . .

As for change of circumstances, mere age alone is not enough, but over time and as kids grow up there are pretty much certain to be changes that justify revisiting physical custody. Transition from elementary to middle school is a common review point.