While you may be able to drive in Canada at the age of 16 and you can vote at 18 (and drink if you are 18 or 19), you may find it difficult to rent a car until you are 25.
Statistically, drivers under the age of 25 get into more auto accidents than any other age group, car rental agencies often are unwilling to rent you a car. They often restrict what cars you can rent and then charge a particularly high fee as well as requiring that you take the CDW. They also often completely ban drivers until they reach the age of 21, entirely. So, what is someone under 25 to do?
Follow up:
Many of the largest rental companies now have clear policies on the rental of cars to drivers who are 21 to 25. Advantage, Alamo, Avis, Budget, Dollar, Enterprise, Hertz, National and Thrifty all have specific rental policies and you can even ask for a quote based on your age. Generally though you can expect to pay a surcharge of around $25 a day and be limited to cars up to full-size. (They usually won't rent you a van, SUV or luxury car.) There are a few states that have lower minimum ages, like New York and Michigan and you may need to enquire directly for those states since the online rental forms aren't always equipped to handle these differences, the fees may also be different.
All this means that you need to do more research and check all the extra fees that are applicable before reserving a car for rent. Websites that compare rates automatically for you usually don't have an option for underage drivers. So that means that you will have to do the comparisons manually. For example, Alamo's website only allows you to add the underage driver option after quoting you the price.
Check if you are eligible for a corporate or organizational rate. They may have terms that will waive the fee or have a negotiated price that is lower or even waive the fee entirely. (Don't try to tell them you are a member of CARP or AARP! Nice try, people!)
Elsewhere, the best suggestion was can give is to look to smaller agencies or local agencies that may have better policies or rates for drivers under 25. For example Via Route offers special rates for those over 21 and under 25 and they offer a much better deal per day with insurance than many of the majors. Rental agencies that rent older cars are often more flexible, try companies like Rent-A-Wreck where the policies may vary from location to location. An easy way to find local agencies is to go to Google maps for the city you are looking for and do a search for category: Automobile Renting
Essentially, renting under the age of 21 is going to be extremely difficult or nearly impossible. For those under 25 and over 21 it's going to be possible, but expensive. The expense might not be worth it and in the end taking taxis (or even the bus) might be more convenient or cheaper. Calculate the total costs including surcharges and insurance with the costs associated with other transportation.
