It depends on the country and the car rental agency, but in general the minimum age is 21. You will also need to have held your driving licence for a minimum of a year, sometimes more, depending on the destination or the category of the car rented.
In most cases, you will have to pay an additional fee if you are under 25 or over 70 years old, to cover the cost of additional insurance. The amount can vary by car rental company, so it is worth checking several companies.