My 9 month old daughter wants to sleep on her stomach?

Some people do: Some people like to sleep on their back, some on their stomach, some on their side. After a person can roll back and forth on her own, we don't worry about what position such a person sleeps in, regardless of whether she is 9 months, 9 years, or 90 years old. We just let people be. For new or young babies, we have them sleep face-up to achieve a mildly lower incidence of unexplained death (SIDS).