Using Botox before lines show up doesn't make a huge amount of sense. There is no convincing, scientific evidence that preventative Botox works.
What is preventative Botox?
Does preventative Botox actually work?
Who is Botox suitable for?
Botox is a drug and it should be looked at as any other drug. You don't want to be taking it if it's not necessary, because there are side effects.