To rent a car without paying any additional fees, you need to be at least 25 years old. If you are between the ages of 21 and 24, however, most major rental car agencies will rent to you for an additional fee. Many car agencies will limit the vehicles that younger drivers are able to rent, not allowing them luxury cars or expensive SUVs.
The underage driver fee varies depending on the rental company and your rental location, but is usually around $25 per day. If you are over 18, you may rent in New York and Michigan, but will likely pay an additional $100 per day for the underage fee. If you are active duty military on orders or traveling with a corporation, these fees are likely to be waived. Check with the car company you are dealing with to determine their policy.
If you are traveling in a foreign country, maximum age limits may apply due to the country's insurance policies. The maximum age limit will vary by country and rental company. These can be found on the company's website or by phone. The age limit usually starts around age 70, so it is advised that you check ahead of time.