If I as the principal applicant land now in October 2009 but my son as the dependent lands half a year later in March 2010, does his PR residency requirement of 2 out of 5 years starts ticking when he lands (Mar 2010) or when I land (Oct 2009)?
When he lands. Once you are all PR, there is no principal applicant anymore and each family member is responsible to keep their own residency requirements.
