To drive a class A, D, C or F vehicle you must be at least eighteen years old, for Class B or E you must be at least twenty-one years old.
Commercial Vehicle Licences
Yes, you can apply for licences for as many commercial vehicles as you wish and they will be shown on your licence in combination, e.g. if you are qualified for Class A, Class D and Class F your licence will be designated ADF. You have to pass tests for each class separately.
No, there are eight separate licences or licence endorsements for different types of vehicle; it is your responsibility to make sure you have the appropriate licence or endorsement for the vehicle you are driving.
You may well do if your vehicle exceeds certain size and weight limits, for example a horsebox. Check the Ministry of Transportation website for details.
Commercial drivers from other countries must apply for an Ontario driver's licence within sixty days of arrival. If you have a commercial licence from other jurisdictions in Canada, or a Canadian military DND 404 permit, you may be eligible to simply exchange your licence (this does not apply to classes B and E (school buses)).