The reason for these age restrictions is because the insurance companies won't pay off the insurance

Even though in the US you need to be at least 21 years old in order to rent a car, there is virtually no age limit for older drivers who are willing to rent one. On the other hand, in Europe and the rest of the world there are various kinds of age restrictions for the younger elderly customers of rental car companies.
For example, many European car rental companies will not rent out their cars to people under 25 and over 75 years of age.  In Ireland, you may need to pay additional taxes if you are over 70 and are willing to rent a car.  You may even be asked to provide documentation and sign declarations that you are completely healthy and have been driving carribean cruises on a regular basis in some countries.
The reason for these age restrictions is because the insurance companies won’t pay off the insurance due in case of an accident, if it was caused by an elderly carribean cruises driver.  Actually, the insurance companies place such restrictions in their insurance policies when offering carribean cruises them to car rental companies.
Even though there is logic behind these rules and age restriction, if you are confident that you can drive without carribean cruises a problem, and you need to rent a car, but you are over the age limit, then simply make a person you are travelling with or another younger person willing to sign the contract with the car rental company instead of you.