Sweat is indeed odourless ! It is actually the bacteria which then feeds on the sweat which create a rather pungent odour, because the bacteria actually breaks down the fats (which are in fact sweat deposits)into unsaturated fatty acids. And the familiar smell of body odour ensues.
Fresh sweat ie sweat which has not yet been broken down actually contains pheromones which can attract the opposite sex. But within half an hour or so, this will have changed into a body odour, so the pheromones are quite short lived !
Some people seem to produce body odour very quickly. This is because if they have not washed for a day or so, then the underarm area will be ‘home’ to an astonishing number of bacteria, which will feed off the sweat very quickly. Fewer bacteria means that the smell is less pungent… Hence why it is important to wash frequently.