There is a lock-in date for dependent children.
See for example http://www.cic.gc.ca/english/resources/manuals/op/op01-eng.pdf page 18:
Family class: Lock-in (of age) is based on the day that the responsible Case Processing Centre
(CPC) receives a sponsorship application. The documents downloaded to CAIPS from the CPC
will specify the lock-in date.
If age is a factor that makes an applicant admissible, officers should use the applicant's age on
the lock-in date. As long as they are the right age on the lock-in date, they can surpass it before
admission.
See also http://www.cic.gc.ca/english/resources/manuals/op/op02-eng.pdf page 8:
The lock-in of age for dependent children is the day CPC-M receives a completed IMM 1344AE
and correct processing fees. Dependent children must be less than 22 years of age when the
sponsorship application is received.
and page 15:
5.13.
Who qualifies as a dependent child?
R2 and Section 6 in this chapter describe who qualifies as a dependent child.
In order to meet R2(b)(i) of the definition of “dependent child”, a child must be under 22 years of
age and not a spouse or common-law partner on the date when the IMM 1344AE is received by
CPC-M, and not a spouse or common-law partner when the visa is issued, and when they arrive
in Canada.
