You can actually still rent if you aren't 25, you just usually pay an extra fee. Found that out one year when our friend's car broke down on the way home from Coachella. I was able to rent a car when I was only 21.
Just to point out, if you were able to pay a fee to still rent a car even though you were under 25, it is probably worth noting that each driver under 25 would likely have to pay the fee in order to actually be an authorized driver for the car... Otherwise, if you were an unauthorized driver and you had the misfortune of being involved in an accident, the authorized driver could potentially get screwed pretty hard.
I think rental car insurance only covers the liability of authorized drivers, and if they allow someone else to drive the car and an accident happens then they have to pay the full cost of repair, with no help from insurance. Not sure though and it might vary based on the specific terms and conditions of the rental company, but I would put your 0L legal skills to good use and read that fine print. There's my absolutely speculative two cents.
