Hertz never had that exclusion with the underage coupon before. My guess is it's one of three things:

1) A mistake. Whoever typed that may have been used to doing so for most other coupons.

2) A new trial balloon, to see if they can still get young renters at similar frequencies as before, but this time paying the underage fee. If so, they are mistaken. Hertz rates tend to be higher than competitors. When there is no underage fee, higher rate + no fee can still be cheaper than lower rate + underage fee. But higher rate + fee >> lower rate + similar fee. Especially when 35k mi Hertz car ~= 35k mi Enterprise, Fox, or Rent a Wreck car. Duh.

But Hertz doesn't always get it. In fact, they tried something like this a few years ago. They eliminated the underage coupon for a few months, presumably thinking Hertz demand is inelastic. Never mind that many retail customers, and young customers especially, have been conditioned to assume that Hertz is always expensive. Many don't even bother checking Hertz's rates before gravitating towards discount brands.

Many underage customers stopped renting, in favor of their old ways of doing things: Enterprise, mass transit, their own car, etc. Some of these, at least judging by the comments on Hertz's Facebook page at that time, had worked their way up to high status, before coming to a sudden stop. At some point, Hertz got the message and reinstated the underage coupon.

3) Hertz suddenly and miraculously has a very large number of new customers, surprisingly mostly in the NY metro area and nowhere else. So in NY Hertz doesn't need underage customers anymore, and since younger drivers pose at least a moderately higher risk, don't really want them to begin with.

My advice in the interim would be to use free Gold Family, hertz.com/goldfamily. The CDP automatically waives the underage fee. Stack a coupon with it for good measure.

And while you're at it, ask Hertz nice and loud on their Facebook page what's up with the underage coupon. The last time somebody did, the rep didn't even have a clue that such a coupon was out there.