Tennessee Code Title 35. Fiduciaries and Trust Estates § 35-7-121

(1) The minor's attainment of twenty-one (21) years of age; provided, that this transfer can be withheld until the minor's attainment of up to twenty-five (25) years of age if the instrument so provides, and if the gift is an inter vivos gift, the instrument further expressly states that deferring termination of custodianship beyond the minor's attainment of twenty-one (21) years of age will cause the transfer to be a gift of a future interest which may have adverse federal and state gift tax consequences; or

(2) The minor's death.