By “naturally”, I’ll assume we aren’t considering disease processes and/or changes to the crystalline lens of the eye (ie. cataract) as these can easily produce significant changes in a short period of time.
It’s not uncommon to see a person under the age of 20 change quite frequently. We have had children come in 3 or 4 times in one year because their prescriptions have changed. Adults, on the other hand, tend to have a bit more stability to their refractive state so rapid changes in their prescriptions require more consideration of alternative causes (ie. diabetes, etc.).