When is the right age to start Botox in Korea?

I am 27 and starting to notice fine lines on my forehead when I raise my eyebrows. A lot of my Korean friends have already started getting Botox as a preventative measure. I am wondering if I should start too or if I am too young. Is preventative Botox actually effective? What is the typical age that people start getting Botox in Korea? I am also concerned about the "frozen" look - I want to keep my facial expressions natural. How much does Botox typically cost in Korean clinics compared to Western countries? Are there any long-term effects I should be aware of?