Is it normal for the perception of odors to change over time? I mean the same odors, like the smell of a snack with the same formula as before.
An interesting question.
It depends how you interpret "perception". Using its most common definition, following cognitive information processing theories, then yes. The perception of odors changes of over time.
This reference is comprehensive about the issue. Specifically about external factors and internal factors (e.g. individual's earlier experiences) on perception (Shiffrin and Schneider, 1977).
A question to pursue could be if there are certain differences among odors that makes some of them more or less susceptible for change in perception over time.
You can further argue that, even if the odor has the same molecules (formula), a person's capabilities of detecting all the molecules will change over time (deterioration), hence changing the perception of the odor over time.
- Shiffrin, R. M., & Schneider, W. (1977). Controlled and automatic human information processing: II. Perceptual learning, automatic attending and a general theory. Psychological review, 84(2), 127.