How do you know how old my mouthwash is?

I cannot determine the age of your mouthwash without specific information, such as the purchase date or expiration date on the bottle. Typically, mouthwash has a shelf life of about 2-3 years from the production date, which is often printed on the packaging. If you have that information, you can assess how old it is. Otherwise, visual signs like changes in color or consistency might also indicate its freshness.