It's kind of confusing. Let me make clear of it to u.
Your age as 21 will be locked at the time of receiving the sponsorship application. You will be accepted as dependent of your mother as long as:
Since sponsorship process will be longer ( About 4 to 5 yrs), you will be 22+ during the process.
During this process, you must be enrolled in a post secondary institution and fully financially dependent on your parents.
OR
If you get married before 22 while the application is in process, since becoming a spouse, you must enrolled in a post-secondary institution as a full time student
AND financially dependent on your mother.
OR
You r 22 yrs old or above, you still need to proof that you continuously has been studying since at the age of 21.
So, one thing is common that You must be continuously studying since your application started processing.