Most Dollar and Thrifty large corporate accounts waive underage fees. Thrifty's small-business-targeted SmartBiz program (with lower/nonexistent minimum revenue commitments) does not, and I can't find a comparable Dollar small-business-targeted program.
Ref.:
http://www.thrifty.com/Programs/Business/SmartBiz.aspx
http://www.dollar.com/Business/D4BMain.aspx
http://www.dollar.com/Business/D4BMa...pRatePlan.aspx
http://www.dollar.com/Business/D4BMa...xRatePlan.aspx
I was never able to find a way to consistently (and legitimately) get the underage fee waived at Dollar or Thrifty when I was under 25. The best I did was negotiate with the agent that I'd buy something like an upgrade, LDW, or prepaid gas if they waived the underage fee. I had reasonable (but not 100%) luck with that. My 25th birthday was one of the happiest of my life. (I rented a car fee-free just to celebrate.
)