Your bank is kind of mixing its metaphors:
* UTMA funds are owned by minors, but I've never heard them lumped in with "minor accounts" before. The money may belong to a minor, but nothing changes when the minor reaches the age of majority; you still have a contract with a different adult. There is no logical rationale for changing the fee schedule when the beneficiary reaches a certain age.
* UTMAs cannot accurately be described as "irrevocable," delivery can be made to the child or the funds can be spent on the child at any time. In any case, if a custodian does not like a new fee, he or she could move the deposit to another bank on receipt of a change in terms notice just as anyone else could.
* The age of majority is 18, not 21. (I'm aware the TUTMA mentions age 21, but it is not the age of majority.)