You can start as early as you like. The ACT sends your best composite score (a perfect score is a 36) to all the colleges you are applying to. That being said, you don't want to overtest so perhaps you can start preparing now and take it once in 10th grade and then twice in 11th grade if necessary.

I think you can start now because on the ACT, you only send your best score.

I started taking it in 9th grade. You only have to send your best score in so there's no harm in starting early.