If the mother is Canadian, how old can her children become Canadian?

The first thing you need to know here is that Canadian immigration law stipulates that to become a Canadian citizen (commonly known as naturalization). Permanent residents (green card holders, you are) need to fill in three of the four years in Canada. This has nothing to do with parents. Your mother became naturalized because she lived for three years, but it has nothing to do with your mother if you want to become naturalized. You have to live for four years, three years.

In addition: if the mother is Canadian, how old can her children follow her to become Canadians?

This question is wrong. It should be that the mother is a Canadian citizen or a permanent citizen, and the children can become permanent residents of Canada under the age of 22, but they cannot become Canadian citizens at once.

You are all green cards, so there is no reunion immigration. Unless you don't meet the residence requirements of the green card (five years and two years), your green card is invalid and you will have no permanent resident status in Canada. At this time, you can apply for reunion immigration before the age of 22; If you are over 22 years old, working or married, you can't apply for reunion immigration.