Essential oils are substances sensitive to heat, light, temperature variations and changes in atmospheric pressure. Some aromatic molecules (camphor, menthol) are crystallizable when the temperature is too cold. It is therefore best to keep them in an amber glass container (brown, blue or green), away from light and in a cool (between 15 and 20 degrees) and dry place.
When kept in the optimum conditions, they can last for more than 100 years. The relics found in the tombs of pharaohs are proof of this. Citrus peels are an exception since they are not obtained by distillation but by cold pressing. They are then less stable.
Although they are part of the family pharmacy, the bathroom is not the best place to keep them. Bottles should be kept out of the reach of children.