Hi Lisa, Ora:
The public Health Agency of Canada have some great resources here http://www.phac-aspc.gc.ca/hp-ps/dca-dea/stages-etapes/childhood-enfance_0-2/sids/index-eng.php
The recommendation is that baby are put to sleep on their backs to prevent SIDs. Once they can flip over by themselves (and not just accidentally) then they can choose how they prefer to sleep.
I love this resource http://www.phac-aspc.gc.ca/hp-ps/dca-dea/stages-etapes/childhood-enfance_0-2/sids/ssb_brochure-eng.php
Here is an excerpt that answers your question perfectly "As your baby gets older, he or she will be able to turn over onto his or her stomach by themself, usually at about 5 months. When this happens, you do not have to reposition your baby."