A common question I'm often asked is "how soon should I start Botox?" While this is a non-cookie cutter answer, I like to think of it like this: in our 20s we can begin to lose about 1% collagen a year. That means the skin begins to visibly thin. I always tell my patients that thicker skin bends less than thinner skin. So my first act is usually preventative measures to help them sustain their collagen and increase production with topicals and in-office procedures such as peels and microneedling. If I see active wrinkles in the consultation, then I may suggest beginning Botox in very small quantities. The belief behind this is if you can prevent a crinkle from ever becoming a resting wrinkle, that's successful prevention! 
One perk I find is that my younger population does not need their neuromodulators as often as my older patients. So, while there is no magic answer, there is nothing wrong with getting Botox in your 20s. It's just something that should be done conservatively.