First of all, in order to rent a car you need a valid driving licence and preferably at least one year of driving experience (though the latter is not obligatory).
The minimum age to rent a car is 20, but the age regulation may vary by car category. Drivers under the age of 25 may incur a young driver surcharge, due to their inexperience in driving.
Keep in mind that most of the car rental companies may require a credit card, or you will have to pay a high amount in cash. Before paying, make sure you have checked the car and the contract. Take pictures of the car if necessary in order to prove that possible damage was already there.
Also be prepared to show proof of your identity when renting a car, so take your driving licence and your passport with you, as a driving license is not a proof of identity in Finland.