You can start anti-wrinkle treatment for your forehead wrinkles as soon as they start to appear! Some practitioners recommend getting it early to stop lines from becoming worse and requiring a higher dose of botulinum toxin later in life.
Be aware that if your wrinkles are particularly deep, botulinum toxin alone may not be effective.
A good practitioner will recommend other products or treatment modalities to try out.
The use of botulinum toxin for cosmetic reasons is illegal for those under 18.