My younger brother is 21 right now but his locked in date was 18 I believe.
My current age is 25 and my locked in date was 21.
So we both were included in the PR Eligibility Letter. And in EL Letter it said that "Any accompanying dependants are eligible to apply for an open work permit, a study permit or a Temporary Resident Visa if the child is not of school age. You however did not submit temporary resident applications for (my younger brother) at the time you applied. You must now apply online. For more information on these permits, please click on this link."
Also if you see dependant age on IRCC website it says 22 or less. Here's the link for that too
https://ircc.canada.ca/english/immigrate/sponsor/aod-tool.asp