DD did this from when she could roll over and has slept on her front from about 7/8 months, we alwasy put her on her back but thats how she wants to sleep, as long as they are capable of rolling back over then I think its' OK but sure someone with more experience will come along.
my ds also sleeps on his tummy and has done from around 6 months. The official advice from the SIDS website is this:
If your baby is less than six months old and you find him sleeping on his tummy, gently turn him onto his back. Dont feel you need to keep getting up all night to check on this. After six months, babies can usually roll onto their backs themselves, so leave him to find his own position. Whatever your babies age, always place them to sleep on the back.