Hi Matt
Generally speaking you have to be over 21 and have at least 3 years driving experience since passing your test. Sometimes the age limit is relaxed. If you book in advance expect to pay about 40 euros per day for a small economic car (a kia picanto or similar), but this price will reduce if you book for more than one day. The cars normally come full of fuel and you return it full, otherwise you will be charged for the full tank. Depending on the company, a small charge (normally 3 or 4 euros) will be added to pay for comprehensive insurance. This does not normally however, cover damage to the tyres, windows or the underneath of the car.
Have fun and safe driving.
Chris