The short version
15 years from month of manufacture
Canada's threshold is 15 years rather than 25, which opens up considerably newer kei trucks than are legal in the United States.
Shorter wait than the USA
A vehicle 15 or more years old is outside the scope of the Motor Vehicle Safety Act requirements that apply to newer imports, and is generally not processed through the Registrar of Imported Vehicles.
Measured by month
As in the USA, eligibility advances month by month from the date of manufacture.
Provincial registration is separate
Federal admissibility does not decide road use. Provinces and territories set their own registration and inspection rules, and they are not uniform on kei vehicles.
Provincial inspection
Most provinces require a safety inspection before a first registration. Requirements vary.
Who decides
Transport Canada and the Canada Border Services Agency; your provincial or territorial registry.
This page is a plain-language summary for orientation. It is not legal or customs advice, and it can go out of date. The authorities named above are the only reliable source for your own situation.