Just got back from Spain and rented a car. Twas my first European driving experience...so here's some quick tips...
1. Not sure about the age thing (I'm over 25 so sorry..didn't pay attention).
2. If you have a valid U.S. driver's license and passport, that's all you need to rent a car...no int'l driver's license necessary.
3. By default, rental cars are stick. If you want automatic, I think you can request one. Again no problem for me - nor you - but can your 25 year old drive a stick if he must be the primary driver? Keep that in mind when scheduling your rental...
4. As far as rates...the best rate I found was at this web site:
http://www.rentacar-europe.com Someone else on this board referred me to them. They were good, but it would be wise to call the rental agency the day you pick up the car - we had a small problem with address of the pickup location in BCN (the Web site sets up your reservation through a large company - so not all information is always up to date).
Hope that helps...have a great trip!