Great question! Treatment of facial wrinkles with Botox or any of the other 2 neurotoxins (medical term) available in the U.S. (e.g. Dysport and Xeomin) has gone from being a treatment modality to more of a preventive measure. Patient's are not waiting anymore until they are 50 years old to treat their wrinkles. In fact, they are now trying to prevent them from forming.
I often see patients in my practice in their early 20's that are seeking treatment because of noticeable frowns in the upper face. The majority of the young patients that I see are concerned about the famous "11's", which are the vertical frown lines that form in between the brows. These wrinkles are formed by the contracting forces of 2 muscles named "the corrugators" (medical term), which aim to pull the brows down and together. This facial expression is commonly done when a person is furious, but in a lot of patients the muscles can be very active even during passive activities such as reading giving the patient the appearance of being mad.
It is never too early to start exploring what Botox is all about. This being said, I would only consider Botox for cosmetic reasons in an adult patient (18 or older). The most important thing is to do your research and find a board certified plastic surgeon with vast experience with injectables in order to minimize any potential complications.
Hope this was helpful. Good luck!