Yeah we thought it was 21 as well, but we went to Moto-Luis in San An, and talked to them and realized that they would let anyone rent a car as long as they had i think 3-4yrs driving experience. We were all 20 and we were able to rent...just go around and talk to places im sure they'll let u if u have enough experience.
My friend and I are both 19 and we were able to rent a car at MotoSud. We called ahead and made a reservation for a jeep and said that we had to be at the airport and were in a hurry. When we got there we already had the cash ready in front of their faces and they never asked us our age or driving experience. They still recorded information off our licenses but thats about it.