Americans and individuals outside Canada can obtain Canadian citizenship if they’re born to Canadian citizens of the first generation. They can become Canadian citizens by descent.
These individuals can also apply for Proof of Citizenship as Canada’s government also invites them for the same. Irrespective of your age and whether your Canadian parent is alive or not, you can still use this process. There are no existing time restrictions on the time duration when a parent leaves Canada and when you apply. Lack of time restrictions implies that even if your parent left Canada years ago, you can still obtain Canadian citizenship, provided you meet all the other essential criteria.
According to IRCC (Immigration, Refugees, and Citizenship Canada), applicants need to prove that their biological and legal parents held Canadian citizenship at the time of their birth. The birth certificate, Card of Canadian citizenship, and citizenship certificate of the Canadian parent are evidence that you will need to provide to IRCC.
If your application is successfully submitted to IRCC, you will receive an acknowledgment receipt. IRCC will also send you a Canadian citizenship certificate once your application gets approved.
It will be great if you contact an expert immigration lawyer or even a consultation agency to get you through the application process. They will help you by checking your documents and application thoroughly. Doing so will save your time and energy.