There isn't one uniform rule for the minimum age to rent a car in Europe. Each rental company has its own policies/rules. Some companies rent to those under 25 but charge extra. Because there's no uniform rule, it's necessary for you to determine the rule of each rental company you're considering. If you click around on their website, you probably can find the info you need. However, if you're going through a consolidator such as AutoEurope, it may be difficult to know, before you confirm the deal, which rental company the consolidator will end up putting you with--I don't know how that works, you'd think the consolidator would ask the driver's age as part of the online sign up.