Americans are not required by law to obtain an international driving permit to drive a rental car in Portugal. Short term visitors staying less than six months in the country can drive their rental with a valid US license.
The rental car age limit in Italy is typically a minimum of 18, but some suppliers will not rent to anyone under 21. There are no maximum age limits but any drivers under 24 will usually have to pay around an additional €20 per day.
Here are some things to keep in mind if you will be buying a car in Portugal: On one hand, having a car gives you the freedom to explore the country at your own pace. You can also avoid crowded buses and trains. However, keep in mind that gas is expensive in Portugal and parking can be difficult to find (and costly).