The eligibility requirements to become a Canadian citizen vary depending on your situation.
Standard eligibility requirements are set for an adult (at least 18 years of age) permanent resident of Canada. Spouses of Canadian citizens also must meet the same eligibility requirements for adults as they are not yet considered as Canadian citizens. Current or former members of the Canadian Armed Forces (CAF) may be eligible to apply through a fast-track process. They must meet the standard qualifications to resume citizenship with the exception that their length of service in, or with, the CAF will be considered instead of the length of time they lived in Canada. Former Canadian citizens who wish to get back Canadian citizenship and adopted children born outside Canada also have separate conditions to meet to become Canadian citizens.

1. Standard eligibility requirements
Conditions in all areas (age, permanent resident status, time lived in Canada, income tax filing, intent to reside, language skills, how well you know Canada and prohibitions) must be met to be eligible to become a Canadian citizen.

  • Age

You must be at least 18 years old.
If you are applying for citizenship for a child under 18 who is a permanent resident of Canada, you must be the child’s parent, adoptive parent or legal guardian and one parent must be a Canadian citizen or apply to become a citizen at the same time (this also applies to adoptive parents).

  • Permanent resident status

You must have permanent resident (PR) status in Canada, have no unfulfilled conditions related to that status, and your PR status must not be in question. This means you must not be under review for immigration or fraud reasons or under a removal order (an order from Canadian officials to leave Canada).
A PR card is not necessary to apply for citizenship.

  • Time lived in Canada

You must have been physically present in Canada as a permanent resident for at least 1,460 days during the six years immediately before the date of your application. You must also be physically present for at least 183 days during each of four calendar years that are fully or partially within the six years immediately before the date of application. These requirements do not apply to children under 18.
Exceptions to these requirements apply for certain Crown servants and certain family members of Crown servants.
When calculating how long you have lived in Canada, you can only count time spent after you became a permanent resident of Canada.

  • Income tax filing

You must have met your personal income tax filing obligations in four taxation years that are fully or partially within the six years immediately before the date you apply.

  • Intent to reside

You must declare your intent to either reside in Canada, work outside Canada as a Crown servant, or live abroad with certain family members who are Crown servants.
Once you become a Canadian citizen, you have the right to enter, remain in, or leave Canada, one of the basic rights of citizenship.

  • Language abilities

Canada has two official languages—English and French. To become a citizen, you must show that you have adequate knowledge of one of these languages by being able to take part in simple conversations, understand basic instructions, questions and directions, use basic grammar and have the sufficient vocabulary to express yourself.

If you are 14 to 64 years of age, you must send documents with your citizenship application that prove you can speak and listen in English or French at this level.

It will also be noted how well you communicate to staff or a citizenship officer during your interview. A citizenship officer will make the final decision on your application, including how well you can communicate in English or French.

  • How well you know Canada

To become a citizen, you must understand the rights, responsibilities and privileges of citizenship, such as voting in elections and obeying the law. You must also show, in English or French, that you understand Canada’s history, values, institutions and symbols.

If you are 14 to 64 years of age, when you apply for citizenship, you will need to take a citizenship test to show you have adequate knowledge of Canada and the responsibilities and privileges of citizenship. It is usually a written test, but it is sometimes taken orally with a citizenship officer.

  • Prohibitions

If you have committed a crime in or outside Canada, you may not be eligible to become a Canadian citizen for a period of time.
Time in prison or on parole does not count as time you have lived in Canada. Time on probation also does not count if you were convicted of a crime.

2.    Fast-Track Process

The Government of Canada recognizes the importance of the Canadian Armed Forces (CAF) and their commitment to defending the values and interests of Canada. The fast-track provision will accelerate citizenship for permanent residents serving in the CAF and foreign military members on exchange with the CAF.

Those who have served for one year less than the standard residence requirement for citizenship may qualify for a grant of citizenship, provided they meet all other requirements. This means the length of time they served in the Canadian Armed Forces is used instead of the length of time they lived in Canada. To be eligible, current or former members of the CAF or foreign military members who were attached or were seconded to the CAF must have completed three years (1,095 days) of service in or with the CAF in the six years (2,190 days) immediately before the date of their application. They also must have met their personal income tax filing obligations in three taxation years that are fully or partially within the six years immediately before the date they applied. Foreign military members attached or seconded to the CAF do not need to be a permanent resident and do not need to meet the tax filing obligation to apply.

Those who are no longer members of the CAF must have been released honourably to be eligible to apply using the fast-track process.

3.   Eligibility requirements for former Canadian citizens who wish to get back Canadian citizenship:

The following are the requirements to be eligible to resume Canadian citizenship:

  • Must have been a Canadian citizen
  • Must have become a permanent resident of Canada after loss of Canadian citizenship
  • Must have no unfulfilled conditions relating to status as a permanent resident
  • Must have been physically present in Canada as a permanent resident for at least 365 days in the two years immediately before application
  • Must have met personal income tax filing obligations for the taxation year immediately before application, and
  • Must have the intention, if granted citizenship, to live in Canada, work outside Canada as a Crown servant, or live abroad with certain family members who are Crown servants.

Former Canadian citizens may no longer be eligible to get back Canadian citizenship if their citizenship has been revoked or taken away, are under a removal order (an order from Canadian officials to leave Canada) or are prohibited from being granted citizenship.

4.   Eligibility requirements for adopted children born outside Canada

The adopted person may be eligible for a direct grant of Canadian citizenship if he/she is not a Canadian citizen, has at least one (1) adoptive parent who, at the time of their adoption, was or is a Canadian citizen, is not subject to the first generation limit to citizenship by descent (unless meets one of the exceptions stated below), and meets the Citizenship Act requirements. Also, the adopted person must fulfill all the requirements for intercountry adoption.

The First Generation limit

The first generation limit prevents children born outside Canada and adopted by a Canadian citizen to be eligible for a grant of Canadian citizenship if their adoptive Canadian citizen parent was born outside Canada to a Canadian citizen or, if their adoptive Canadian citizen parent was granted Canadian citizenship under section 5.1, the adoption provisions of the Citizenship Act. disallow

However, there are two exceptions to the first generation limit:

  • at the time of the person’s adoption, either of the person’s adoptive parents was employed outside Canada in or with the Canadian Armed Forces, the federal public administration or the public service of a province or territory, other than as a locally engaged person (a Crown servant);
  • at the time of either of the adoptive parents’ birth or adoption, one of their parents (the adopted person’s grandparents) was employed outside Canada in or with the Canadian Armed Forces, the federal public administration or the public service of a province or territory, other than as a locally engaged person (a Crown Servant).

Requirements for intercountry adoption

Adopting a child under 18 years of age

For a child adopted by a Canadian citizen to be granted citizenship, the adoption must be in the best interests of the child and will create a genuine relationship of parent and child. As such, the primary reason for adoption must not be to acquire status or privilege in relation to immigration or citizenship.  It must also be in accordance with international adoption laws, the laws of the place where the adoption took place and the laws of the country of residence where the adoptive parents reside.

Adopting a person 18 years of age or older

For a person over 18 adopted by a Canadian citizen to be granted citizenship, there must have already been a genuine relationship of parent and child before the person turned 18 and at the time of adoption. As such, the primary reason for adoption must not be to acquire status or privilege in relation to immigration or citizenship. Also, the adoption must be in accordance with international adoption laws, the laws of the place where the adoption took place and the laws of the country of residence where the adoptive parents reside.

Reference: http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=355&top=5