You need to refer to your State law. Only the legal custodian may release the funds to the minor, even if they have attained the age for the funds to be released. The minor needs to talk to their own legal counsel to have a valid custodian appointed to release the funds and provide the bank the proper paperwork indicating the appointment of the successor custodian. At that point, you should provide that paperwork to your legal counsel to make sure all is in order and then release the funds.
For example, in Texas:
Sec. 141.019. RENUNCIATION, RESIGNATION, DEATH, OR REMOVAL OF CUSTODIAN; DESIGNATION OF SUCCESSOR CUSTODIAN.
(d) If a custodian is ineligible, dies, or becomes incapacitated without having effectively designated a successor and the minor is at least 14 years of age, the minor may designate as successor custodian an adult member of the minor's family, a guardian of the minor, or a trust company in the manner prescribed by Subsection (b). If the minor is younger than 14 years of age or fails to act within 60 days after the ineligibility, death, or incapacity of the custodian, the minor's guardian becomes successor custodian. If the minor has no guardian or the minor's guardian declines to act, the transferor, the legal representative of the transferor or of the custodian, an adult member of the minor's family, or any other interested person may petition the court to designate a successor custodian.