i always had to rent a car for work, and i was under 25. but then again, it ws under my employers contract, so that might be why i didn't have an issue. you probably just have to pay more - but just check out the company's website or call to get the scoop (hertz is my fav, or national, budget, etc.).
little tip ... find out the lowest per day price on something like expedia.com - then go to priceline and name your own price - i always get a pretty good deal that way. and i use that when not renting for work.