Babies are apparently born wiyth a sesnse of smell: they turn their faces towards breastpads dampened with their mothers' breatsmilk, and turn away from nasty smells like ammonia. However, I doubt whether they become conscious of smells for a few years. Certainly my elder two didn't seem to notice smells - or even to know how to smell things - until they were at least 4. Until then they didn't notice poo smells or farts at all. Generally, also, girls have a far more sensitive and discriminatory snese of smell than boys.