I'd play it safe and forget the past 10 years bit, just go with if over 18 for at least 6 months. Classic CIC inconsistent information can be seen in these two examples:
PCC needed if 6 months, over 18, in past 10 yrs: http://www.cic.gc.ca/english/information/applications/guides/pdf/PC_LetterB_e.pdf
PCC needed if 6 months, over 18: http://www.cic.gc.ca/english/information/applications/guides/pdf/PC_LetterA_e.pdf
I've seen the same varying info on this topic at other places on the CIC website.