Which applies no restrictions but gives you an incentive to use it only when you think you need it (as opposed to all the time, just to be safe).

Give "barrier use" as a spell, start it at 100%, and give it a possibility of decreasing each time the spell is used (i.e. opposite to how a skill would normally improve).

To stop longer lived characters being at a disadvantage, give some % back when someone reaches an older age bracket.