It also depends on the jeans. Some jeans are, for some reason, more likely to smell after being worn just once. I have no idea why, but several people I know have corroborated this independently.
One thing that can affect this is the material used in the jeans. Typically, a lot of synthetic fabrics tend to start smelling more easily, while wool and silk are known for being naturally odor resistant. This can vary some, but it's a good general guideline.
I think it's past time for another Stupid Questions thread, so here we go.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Please respect people trying to fix any ignorance they might have, rather than mocking that ignorance.