In case anyone else needs to know, there is a 6 Euro per day surcharge for renters under 25 in Amsterdam. Minimum renter's age is 21 and maximum is 75.

Compare that 6 Euro/day with $25/day I pay in the states and it's a good deal. This from National Car Rental by the way.
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-Rob Riccardelli
80GB 16MB MK2 090000736