Plastic bottles like these are made of materials that aging over time.
There are two main types of plastics which make up most plastic water bottles available today; polyethylene terephthalate (PET) and high-density polyethylene (HDPE). The former is less costly, thinner, and more flexible–meaning it can be used for clear or translucent containers with labels on them. HDPE is thicker, denser, and less elastic than PET–so it’s usually found in opaque containers without labeling-like milk jugs or recyclable soft drink bottles.
These different properties will affect the rate at which the bottle will age under various conditions.