To rent cars from most car rental agencies in the Canadian Rockies you need
major credit card, hold a valid drivers licence (the names must match),
and be over 21 years old. Some agencies allow you to "cash qualify"
but the paperwork is extensive and is not viable for out of town visitors
(check when you make your reservation). TIP: If the person who's name is on the
credit card is not the driver, then add the real driver as a second driver
(there may be a small charge for that).
There are plenty of rental agencies in the Rockies, as well as in downtown Calgary, at the airport arrivals level. If you got to the mountains with a tour group and want a little independence, ask your hotel's front desk to make the arrangements.
Many agencies have "weekend specials" but reserve a few days before long weekends. Stampede Week (July) is the busiest week of the year--reserve your vehicle well in advance.
You may be asked if you are heading to the mountains, so you have winter gear in the car, and have the right tires for mountain driving--it can snow in Banff any month of the year. TIP: If you are getting a 4-cylinder subcompact, it will drive much better with a standard - not automatic - transmission (if you're used to that).
Better Business Bureau tips.
Find Local Car Rental Offices.